Minimum Time to Visit a Cell In a Grid
10 Prerequisite LeetCode Problems
“2577. Minimum Time to Visit a Cell In a Grid” requires understanding of BreadthFirst Search (BFS), and handling of gridbased problems with some movement constraints.
“200. Number of Islands”:
 This problem is a good start to understand the BFS approach in the context of a grid.
“994. Rotting Oranges”:
 This problem is similar to the problem at hand in that it also involves grid traversal with a BFS. It introduces the concept of time steps with each movement.
“542. 01 Matrix”:
 This problem also involves BFS on a grid, with the task to find shortest distances from certain cells.
“1091. Shortest Path in Binary Matrix”:
 This problem is about finding shortest paths in a grid using BFS, similar to “2577. Minimum Time to Visit a Cell In a Grid”.
“127. Word Ladder”:
 This problem requires understanding of BFS in a graph context.
“207. Course Schedule”:
 This problem can help you understand topological sort, which is a variation of BFS, although not directly related, but good to understand as part of BFSbased problems.
“785. Is Graph Bipartite?”:
 This problem is about BFS in a graph with some constraints on the traversal, which could be beneficial in understanding “2577. Minimum Time to Visit a Cell In a Grid”.
“102. Binary Tree Level Order Traversal”:
 This problem provides a good example of BFS in a binary tree context.
“733. Flood Fill”:
 This problem is another gridbased BFS problem where you have to ‘flood’ a region of the grid.
“279. Perfect Squares”:
 This problem is an example of BFS in the context of number theory. It shows how BFS can be used to solve nongraph related problems.
These cover BFS and gridbased problems, which are essential for solving “2577. Minimum Time to Visit a Cell In a Grid”.
Bard’s response:
Here are 10 problems as preparation for tackling Minimum Time to Visit a Cell In a Grid:
For the problem Minimum Time to Visit a Cell In a Grid, the following is a good preparation:
 Unique Paths
 This problem is similar to Minimum Time to Visit a Cell In a Grid in that it involves finding the number of possible paths to reach a target cell in a grid. The difference is that the paths in Unique Paths cannot intersect.
 Robot in Grid
 This problem is similar to Minimum Time to Visit a Cell In a Grid in that it involves moving an agent through a grid. The difference is that the agent in Robot in Grid can only move in four directions: up, down, left, and right.
 Flood Fill
 This problem is similar to Minimum Time to Visit a Cell In a Grid in that it involves filling a grid with a certain color. The difference is that the grid in Flood Fill is not rectangular.
 Maze Solver
 This problem is similar to Minimum Time to Visit a Cell In a Grid in that it involves finding a path through a maze. The difference is that the maze in Maze Solver can be more complex than the grid in Minimum Time to Visit a Cell In a Grid.
 Rat in Maze
 This problem is similar to Minimum Time to Visit a Cell In a Grid in that it involves finding a path through a maze. The difference is that the rat in Rat in Maze can only move in four directions: up, down, left, and right.
 Word Search
 This problem is similar to Minimum Time to Visit a Cell In a Grid in that it involves finding a path through a grid. The difference is that the grid in Word Search contains letters, and the goal is to find a path that spells out a certain word.
 Word Ladder
 This problem is similar to Minimum Time to Visit a Cell In a Grid in that it involves finding a path between two words. The difference is that the path in Word Ladder must consist of words that are related to each other.
 Jump Game
 This problem is similar to Minimum Time to Visit a Cell In a Grid in that it involves finding the minimum number of steps required to reach the end of a grid. The difference is that the grid in Jump Game can have obstacles.
 Jump Game II
 This problem is similar to Jump Game in that it involves finding the minimum number of steps required to reach the end of a grid. The difference is that the grid in Jump Game II can have negative values.
 Minimum Path Sum
 This problem is similar to Minimum Time to Visit a Cell In a Grid in that it involves finding the minimum cost to reach a target cell in a grid. The difference is that the cost of moving from one cell to another is not constant in Minimum Path Sum.
Identifying Problem Isomorphism
This can be mapped to the problem Minimum Path Sum
. The two problems are isomorphic because they both involve finding the minimum cost to reach a target cell in a grid. The difference is that the cost of moving from one cell to another is not constant in the minimumTime
problem.
The minimumTime
problem is simpler than the Minimum Path Sum
problem because the cost of moving from one cell to another is constant in the minimumTime
problem. This means that the minimumTime
problem can be solved using a simpler algorithm.
Here is a list of other isomorphic problems, arranged from simplest to most complex:
 Unique Paths
 Robot in Grid
 Flood Fill
 Maze Solver
 Rat in Maze
 Knight’s Tour
 Word Search
 Word Ladder
Problem Classification
This problem pertains to Graph Theory and Path Finding.
Problem Components:
Problem Input:
 A m x n matrix grid with nonnegative integers, where each integer represents the minimum time required to be able to visit the corresponding cell.
 The dimensions m and n, where 2 <= m, n <= 1000 and 4 <= m * n <= 105.
Constraints:
 The grid’s values range from 0 <= grid[i][j] <= 105.
 The topleft cell’s value, grid[0][0], is always 0.
 The player can only move in four directions: up, down, left, and right, and each move takes 1 second.
Problem Output:
 The minimum time required to reach the bottomright cell of the matrix from the topleft cell, given the movement constraints.
 The output should be 1 if it is not possible to reach the bottomright cell from the topleft cell under the provided conditions.
This can be further classified as a Shortest Path problem where we have to find the minimum time path from a source cell to a destination cell in a grid. The constraints and conditions of the problem make it a unique and nontrivial variant of the shortest path problem, which would typically involve Breadthfirst Search, Dijkstra’s Algorithm, or a similar graph traversal algorithm.
Understanding the problem’s categorization and components helps in devising an efficient strategy to tackle it. It highlights the underlying concepts needed to devise a solution and assists in the problemsolving process.
The problem is a dynamic programming problem. Dynamic programming is a technique for solving problems by breaking them down into smaller subproblems and using the solutions to the subproblems to solve the original problem.
The domain of the problem is graph theory. Graph theory is the study of graphs, which are mathematical structures that represent pairwise relations between objects. In this problem, the graph is the grid of cells, and the relations between the cells are the possible moves that can be made between them.
The what components of the problem are as follows:
 A grid of cells, where each cell has a nonnegative integer value.
 A starting cell, which is the topleft cell of the grid.
 A target cell, which is the bottomright cell of the grid.
 A set of rules that determine the possible moves that can be made between cells.
The problem can be further classified as a minimum time problem, because the goal is to find the minimum amount of time required to visit the target cell.
The problem can also be classified as a constrained optimization problem, because there are constraints on the possible moves that can be made.
The problem is a dynamic programming problem in the domain of graph theory. The problem can be further classified as a minimum time constrained optimization problem.
Here are some additional thoughts on the problem statement:
 The problem is welldefined, because the input and output are clearly specified.
 The problem is solvable, because it can be solved using dynamic programming.
 The problem is nontrivial, because it is not immediately obvious how to solve it using dynamic programming.
 The problem is interesting, because it is a realworld problem that has applications in a variety of fields.
Distilling the Problem to Its Core Elements
The fundamental concept or principle this problem is based upon is dynamic programming. Dynamic programming is a technique for solving problems by breaking them down into smaller subproblems and using the solutions to the subproblems to solve the original problem. In this problem, the subproblems are the paths that can be taken from the starting cell to the target cell. The solutions to the subproblems are the minimum amount of time required to take each path.
The simplest way I would describe this problem to someone unfamiliar with the subject is as follows:
You are given a grid of cells, where each cell has a nonnegative integer value. You start at the topleft cell of the grid and you need to reach the bottomright cell. You can only move to adjacent cells, and each move takes one second. The goal is to find the minimum amount of time required to reach the bottomright cell.
The core problem we are trying to solve is to find the minimum amount of time required to reach the bottomright cell. We can simplify the problem statement by saying that we need to find the minimum path from the starting cell to the target cell.
The problem can be broken down into the following key components:
 The grid of cells
 The starting cell
 The target cell
 The set of possible moves
 The minimum amount of time required to reach the target cell
 The minimal set of operations we need to perform to solve this problem are:
 Initialize a table to store the minimum amount of time required to reach each cell.
 Fill the table starting from the starting cell.
 The table can be filled using dynamic programming.
 The minimum amount of time required to reach the target cell is the value stored in the table at the target cell.
Visual Model of the Problem
Here is a way to visualize the problem statement:


This code will print out the grid of cells, as well as the starting cell and the target cell. This can be helpful for visualizing the problem and understanding the different subproblems that need to be solved.
Here is an example of how the code would be used:


This code would print out the following:
0 1 3 2
5 1 2 5
4 3 8 6
Starting cell: (0, 0)
Target cell: (2, 3)
This visualization can be helpful for understanding the problem and the different subproblems that need to be solved. For example, the subproblem of finding the minimum path from the starting cell to the cell at (1, 1) can be visualized as follows:


This code would print out the following:
0 1 3 2
5 1 2 5
4 3 8 6
Starting cell: (0, 0)
Target cell: (1, 1)
This visualization can be helpful for understanding the different possible moves that can be made from the starting cell to the target cell.
Problem Restatement
You are given a grid of cells, where each cell has a nonnegative integer value. You start at the topleft cell of the grid and you need to reach the bottomright cell. You can only move to adjacent cells, and each move takes one second. The goal is to find the minimum amount of time required to reach the bottomright cell.
The essential elements of the problem are:
 A grid of cells
 A starting cell
 A target cell
 A set of possible moves
 The minimum amount of time required to reach the target cell
The requirements of the problem are:
 The grid of cells is a 2D array of nonnegative integers.
 The starting cell is the topleft cell of the grid.
 The target cell is the bottomright cell of the grid.
 The set of possible moves is the set of all adjacent cells to a given cell.
 The minimum amount of time required to reach the target cell is the minimum number of seconds required to reach the target cell from the starting cell.
The constraints of the problem are:
 The only way to move between cells is to move to an adjacent cell.
 Each move takes one second.
 The goal is to find the minimum amount of time required to reach the target cell.
Abstract Representation of the Problem
The abstract representation of the problem can be described as follows:
 There is a set of states, which are the cells in the grid.
 There is a start state, which is the topleft cell of the grid.
 There is a goal state, which is the bottomright cell of the grid.
 There is a set of transitions, which are the possible moves that can be made between cells.
 The cost of a transition is the time required to make the move.
 The goal is to find a path from the start state to the goal state with the minimum cost.
The abstract representation of the problem emphasizes the structure and key elements of the problem, without the specific realworld details. This makes the problem easier to understand and solve, because it focuses on the essential parts of the problem.
Here is an example of how the abstract representation of the problem can be used to solve the problem:
 We can create a table to store the minimum cost of reaching each state from the start state.
 We can fill the table using dynamic programming.
 The minimum cost of reaching the goal state is the value stored in the table at the goal state.
Terminology
Here are some specialized terms, jargon, and technical concepts that are crucial to understanding this problem or solution:
 Dynamic programming is a technique for solving problems by breaking them down into smaller subproblems and using the solutions to the subproblems to solve the original problem.
 State is a situation or condition that a system is in. In this problem, a state is a cell in the grid.
 Start state is the initial state of a system. In this problem, the start state is the topleft cell of the grid.
 Goal state is the desired state of a system. In this problem, the goal state is the bottomright cell of the grid.
 Transition is a change from one state to another. In this problem, a transition is a move from one cell to another.
 Cost is a measure of the value or importance of something. In this problem, the cost of a transition is the time required to make the move.
 Path is a sequence of states that leads from the start state to the goal state.
 Minimum cost is the smallest possible cost of a path from the start state to the goal state.
These terms are crucial to understanding the problem because they define the essential elements of the problem and the solution. For example, the term “dynamic programming” is used to describe the technique that is used to solve the problem. The terms “state”, “start state”, and “goal state” are used to describe the different states that the system can be in. The terms “transition” and “cost” are used to describe the moves that can be made between states and the cost of making those moves. The terms “path” and “minimum cost” are used to describe the solution to the problem.
Problem Simplification and Explanation
The key concepts involved are:
 Grid: A grid is a twodimensional array of cells. In this problem, the grid represents a map of the area that you need to traverse.
 Cells: The cells in the grid represent different locations in the area. Each cell has a nonnegative integer value, which represents the time required to reach that cell from the starting cell.
 Starting cell: The starting cell is the cell that you start in. In this problem, the starting cell is the topleft cell of the grid.
 Target cell: The target cell is the cell that you need to reach. In this problem, the target cell is the bottomright cell of the grid.
 Minimum time: The minimum time is the shortest amount of time that it takes to reach the target cell from the starting cell.
The key concepts interact as follows:
 The grid defines the possible paths that you can take to reach the target cell.
 The cells in the grid represent the time required to take each path.
 The starting cell is the starting point of your journey.
 The target cell is the destination of your journey.
 The minimum time is the goal of your journey.
Here is a metaphor that can help you understand the problem better:
 Imagine that you are in a maze. The maze represents the grid, and the walls of the maze represent the cells in the grid. The starting cell is where you start in the maze, and the target cell is the exit of the maze. The minimum time is the shortest amount of time that it takes you to reach the exit of the maze.
Constraints
Here are some specific characteristics or conditions that can be exploited to our advantage in finding an efficient solution to the problem:
 The grid is a 2D array, which means that it can be stored in a data structure that is optimized for 2D arrays.
 The cells in the grid have nonnegative integer values, which means that we can use simple arithmetic operations to calculate the minimum time required to reach the target cell.
 The minimum time required to reach the target cell is a subproblem of the original problem. This means that we can solve the problem by recursively solving the subproblems.
 The subproblems are independent of each other. This means that we can solve the subproblems in parallel.
These characteristics and conditions can be exploited to our advantage by using a dynamic programming algorithm. Dynamic programming is a technique for solving problems by breaking them down into smaller subproblems and using the solutions to the subproblems to solve the original problem. In this problem, the subproblems are the paths that can be taken from the starting cell to the target cell. The solutions to the subproblems are the minimum amount of time required to take each path.
The dynamic programming algorithm works by first creating a table to store the minimum amount of time required to reach each cell from the starting cell. The table is filled recursively, starting from the starting cell and working towards the target cell. The minimum amount of time required to reach the target cell is the value stored in the table at the target cell.
The dynamic programming algorithm can be implemented efficiently by using a data structure that is optimized for 2D arrays. The algorithm can also be implemented in parallel, which can further improve the efficiency of the algorithm.
The key insights from analyzing the constraints are:
 The grid is a 2D array. This means that it can be stored in a data structure that is optimized for 2D arrays.
 The cells in the grid have nonnegative integer values. This means that we can use simple arithmetic operations to calculate the minimum time required to reach the target cell.
 The minimum time required to reach the target cell is a subproblem of the original problem. This means that we can solve the problem by recursively solving the subproblems.
 The subproblems are independent of each other. This means that we can solve the subproblems in parallel.
These insights can be used to design an efficient algorithm for solving the problem. The algorithm would need to be able to store the grid in a data structure that is optimized for 2D arrays. The algorithm would also need to be able to perform simple arithmetic operations and recursion. The algorithm could be implemented in parallel to further improve its efficiency.
Here are some additional insights that can be gained from analyzing the constraints:
 The minimum time required to reach the target cell is bounded by the maximum value of the cells in the grid. This means that the algorithm can terminate after a finite number of steps.
 The minimum time required to reach the target cell is decreasing as you get closer to the target cell. This means that the algorithm can focus on the cells that are closest to the target cell first.
These insights can be used to further optimize the algorithm. For example, the algorithm could stop searching for a path once it finds a path that has a minimum time that is less than the maximum value of the cells in the grid. The algorithm could also focus on the cells that are closest to the target cell first.
Case Analysis
Here are some additional examples or test cases that cover a wider range of the input space, including edge and boundary conditions:
 Case 1: The grid is empty. In this case, the minimum time required to reach the target cell is 0.
 Case 2: The grid is a single cell. In this case, the minimum time required to reach the target cell is 0.
 Case 3: The grid is a 2x2 grid with all cells having the value 1. In this case, the minimum time required to reach the target cell is 2.
 Case 4: The grid is a 3x3 grid with all cells having the value 1, except for the cell in the bottomright corner, which has the value 2. In this case, the minimum time required to reach the target cell is 3.
 Case 5: The grid is a 4x4 grid with all cells having the value 1, except for the cells in the bottomright corner, which have the value 2. In this case, the minimum time required to reach the target cell is 4.
These cases highlight different aspects of the problem, such as the minimum time required to reach the target cell, the behavior of the algorithm when the grid is empty or a single cell, and the behavior of the algorithm when the cells in the grid have different values.
The key constraints of the problem are that the grid is a 2D array of nonnegative integer values, and the minimum time required to reach the target cell is a subproblem of the original problem. The potential pitfalls of the problem are that the algorithm may not terminate if the grid is infinite, and the algorithm may not find the optimal solution if the grid is very large.
The expected output for each case is the minimum time required to reach the target cell. The reasoning behind the expected output is that the algorithm works by recursively solving the subproblems. The minimum time required to reach the target cell is the value stored in the table at the target cell.
These cases help to generate key insights about the problem, such as the fact that the minimum time required to reach the target cell is bounded by the maximum value of the cells in the grid. They also help to ensure that the solution is robust and handles all possible scenarios.
I have categorized these cases as follows:
 Empty grid: This case tests the behavior of the algorithm when the grid is empty.
 Single cell: This case tests the behavior of the algorithm when the grid is a single cell.
 Uniform grid: This case tests the behavior of the algorithm when all cells in the grid have the same value.
 Nonuniform grid: This case tests the behavior of the algorithm when cells in the grid have different values.
Here are the key insights from analyzing the different cases:
 The minimum time required to reach the target cell is bounded by the maximum value of the cells in the grid.
 The algorithm terminates after a finite number of steps.
 The algorithm is robust and handles all possible scenarios.
 The algorithm can be optimized by focusing on the cells that are closest to the target cell first.
These insights can be used to improve the efficiency and robustness of the algorithm. For example, the algorithm could be optimized by stopping searching for a path once it finds a path that has a minimum time that is less than the maximum value of the cells in the grid. The algorithm could also be made more robust by handling edge and boundary conditions gracefully.
Here are some additional insights that can be gained from analyzing the different cases:
 The algorithm can be used to solve a variety of problems that involve finding the minimum time required to reach a target from a starting point.
 The algorithm can be extended to handle problems where the cells in the grid have different costs.
 The algorithm can be extended to handle problems where the grid is infinite.
These insights can be used to apply the algorithm to other problems and to further improve the algorithm.
Identification of Applicable Theoretical Concepts
Hhere are some mathematical or algorithmic concepts or properties that can be applied to simplify the problem or make it more manageable:
 Dynamic programming: Dynamic programming is a technique for solving problems by breaking them down into smaller subproblems and using the solutions to the subproblems to solve the original problem. This technique can be applied to the problem of finding the minimum time required to reach the target cell by breaking the problem down into subproblems that represent the paths that can be taken from the starting cell to the target cell.
 Memoization: Memoization is a technique for storing the solutions to subproblems so that they do not have to be recalculated every time they are needed. This technique can be used to improve the efficiency of the dynamic programming algorithm by storing the minimum time required to reach each cell from the starting cell.
 Breadthfirst search: Breadthfirst search is an algorithm for traversing a graph by exploring all of the nodes that are adjacent to a given node before exploring any other nodes. This algorithm can be used to find the minimum time required to reach the target cell by exploring all of the paths that can be taken from the starting cell to the target cell.
 Greedy algorithm: A greedy algorithm is an algorithm that makes the best choice at each step in order to optimize the overall solution. This algorithm can be used to find the minimum time required to reach the target cell by making the best choice at each step in order to minimize the time required to reach the target cell.
These are just a few of the mathematical or algorithmic concepts or properties that can be applied to simplify the problem or make it more manageable. The specific concept or property that is most appropriate for a given problem will depend on the specific characteristics of the problem.
In addition to the mathematical or algorithmic concepts or properties listed above, there are also a number of existing theories, metrics, and methodologies in mathematics, computer science, and related fields that can be applied to calculate, measure, or perform the operations required by the problem statement more effectively or efficiently. For example, the concept of big O notation can be used to measure the asymptotic complexity of an algorithm, and the concept of hash tables can be used to store and retrieve data more efficiently.
The specific theory, metric, or methodology that is most appropriate for a given problem will depend on the specific characteristics of the problem. However, by applying these concepts and methodologies, it is possible to simplify the problem or make it more manageable, and to calculate, measure, or perform the operations required by the problem statement more effectively or efficiently.
Problem Breakdown and Solution Methodology
Here is how I would approach solving the problem:
 Define the problem. The first step is to define the problem clearly. This includes identifying the input and output of the problem, as well as the constraints on the problem.
 Identify the key concepts. The next step is to identify the key concepts that are involved in the problem. This includes understanding the meaning of the input and output, as well as the constraints on the problem.
 Develop an approach. Once the problem and the key concepts have been identified, the next step is to develop an approach to solving the problem. This may involve using a variety of techniques, such as dynamic programming, memoization, breadthfirst search, or greedy algorithms.
 Implement the approach. Once an approach has been developed, the next step is to implement it. This may involve using a programming language, such as Python or Java.
 Test the solution. Once the solution has been implemented, the next step is to test it. This involves running the solution on a variety of input cases and verifying that the output is correct.
 Analyze the results. Once the solution has been tested, the next step is to analyze the results. This involves understanding how the solution works and identifying any potential areas for improvement.
Here is an example of how I would use metaphors, analogies, and visual representations to make my explanation more intuitive:
 I could use the metaphor of a maze to represent the grid. The starting cell would be the entrance to the maze, and the target cell would be the exit. The cells in the grid would represent the walls of the maze.
 I could use the analogy of a staircase to represent the paths that can be taken from the starting cell to the target cell. The steps on the staircase would represent the cells in the grid, and the height of the staircase would represent the time required to reach the target cell.
 I could use a visual representation of the grid to show how the paths can be taken from the starting cell to the target cell. The visual representation could be a simple diagram or a more complex 3D model.
Here is how specific operations or changes in the problem’s parameters would affect the solution:
 If the grid is larger, the solution will take longer to compute.
 If the cells in the grid have different values, the solution will be more complex.
 If the grid is infinite, the solution may not terminate.
Here is how my approach works using one or more example cases:
 Example case 1: The grid is empty. In this case, the minimum time required to reach the target cell is 0.
 Example case 2: The grid is a single cell. In this case, the minimum time required to reach the target cell is 0.
 Example case 3: The grid is a 2x2 grid with all cells having the value 1. In this case, the minimum time required to reach the target cell is 2.
Inference of ProblemSolving Approach from the Problem Statement
There are a few key aspects of the problem statement that suggest that it can be solved using dynamic programming:
 The problem can be broken down into smaller subproblems. In this case, the subproblems are the paths that can be taken from the starting cell to the target cell.
 The solutions to the subproblems can be reused. In this case, the solution to the subproblem of finding the minimum time required to reach a cell can be reused when finding the minimum time required to reach a neighboring cell.
 The problem has an optimal substructure. In this case, the minimum time required to reach the target cell is the minimum of the minimum times required to reach the target cell from each of the neighboring cells.
These three properties are all characteristics of problems that can be solved using dynamic programming. Therefore, it is reasonable to infer that this problem can be solved using dynamic programming.
Here is an example of how dynamic programming can be used to solve the problem:
 Initialize a table to store the minimum time required to reach each cell.
 Fill the table recursively, starting from the starting cell and working towards the target cell.
 The minimum time required to reach the target cell is the value stored in the table at the target cell.
This is just one example of how dynamic programming can be used to solve the problem. There are other possible solutions, but they all rely on the same three properties that make the problem amenable to dynamic programming.
Stepwise Refinement
Stepwise refinement of our approach to solving this problem:
 Define the problem. What is the problem that we are trying to solve? What are the input and output of the problem? What are the constraints on the problem?
 Identify the key concepts. What are the key concepts that are involved in the problem? What do they mean?
 Develop an approach. What is our approach to solving the problem? What techniques will we use?
 Implement the approach. How will we implement our approach? What programming language will we use?
 Test the solution. How will we test our solution? What input cases will we use?
 Analyze the results. How well does our solution work? Are there any improvements that we can make?
How can we take the highlevel solution approach and distill it into more granular, actionable steps?
The highlevel solution approach can be distilled into more granular, actionable steps by breaking it down into smaller subproblems. For example, the subproblems for the problem of finding the minimum time required to reach the target cell could be:
 Finding the minimum time required to reach a cell from its neighbors.
 Filling the table recursively, starting from the starting cell and working towards the target cell.
 Finding the minimum time required to reach the target cell.
Each of these subproblems can then be broken down into even smaller subproblems, until the steps are granular enough to be implemented.
Could you identify any parts of the problem that can be solved independently?
Yes, there are parts of the problem that can be solved independently. For example, the subproblems of finding the minimum time required to reach a cell from its neighbors can be solved independently. This means that we can solve each subproblem in parallel, which can make the overall solution faster.
Are there any repeatable patterns within our solution?
Yes, there are repeatable patterns within our solution. For example, the subproblems of finding the minimum time required to reach a cell from its neighbors are all similar. This means that we can implement a generic solution for these subproblems, which can be reused for different problems.
Solution Approach and Analysis
Here is how I would approach solving the problem:
Define the problem.
The problem is to find the minimum time required to reach the target cell in a grid, given the time required to move from one cell to another.
Identify the key concepts.
The key concepts involved in the problem are:
 The grid: This is a 2D array of cells, where each cell has a nonnegative integer value.
 The time required to move: This is the time required to move from one cell to another.
 The minimum time required to reach the target cell: This is the minimum time required to reach the target cell, starting from any cell in the grid.
Develop an approach.
One approach to solving the problem is to use dynamic programming. Dynamic programming is a technique for solving problems by breaking them down into smaller subproblems and using the solutions to the subproblems to solve the original problem.
In this problem, the subproblems are the paths that can be taken from the starting cell to the target cell. The solution to the subproblem of finding the minimum time required to reach a cell is the minimum of the minimum times required to reach the cell from each of its neighbors.
The overall solution can be implemented as follows:
 Initialize a table to store the minimum time required to reach each cell.
 Fill the table recursively, starting from the starting cell and working towards the target cell.
 The minimum time required to reach the target cell is the value stored in the table at the target cell.
Implement the approach.
The approach can be implemented in a programming language, such as Python or Java. The code would need to initialize the table, fill the table recursively, and then print the value stored in the table at the target cell.
Test the solution.
The solution can be tested by running it on a variety of input cases. The input cases should include cases where the grid is small, cases where the grid is large, and cases where the cells in the grid have different values.
Analyze the results.
The results of the tests can be analyzed to see how well the solution works. The analysis should focus on the following:
 The accuracy of the solution: Does the solution correctly find the minimum time required to reach the target cell?
 The efficiency of the solution: How long does it take the solution to run on different input cases?
 The scalability of the solution: How well does the solution work as the size of the grid increases?
Visual representations.
The solution can be visualized by creating a graph that shows the paths that can be taken from the starting cell to the target cell. The graph can be colored to show the minimum time required to reach each cell.
Specific operations or changes in the problem’s parameters.
The solution would be affected by the following specific operations or changes in the problem’s parameters:
 The size of the grid: The solution would take longer to run as the size of the grid increases.
 The values of the cells in the grid: The solution would be more complex if the cells in the grid have different values.
 The constraints on the problem: The solution would be different if there are constraints on the paths that can be taken.
Example cases.
The solution can be demonstrated by running it on the following example cases:
 A 2x2 grid with all cells having the value 1.
 A 3x3 grid with all cells having the value 1, except for the cell in the bottomright corner, which has the value 2.
 A 4x4 grid with all cells having the value 1, except for the cells in the bottomright corner, which have the value 2.
Thought Process
Here is the thought process I would use to solve the problem and the code in Python3:
Thought process:
Define the problem.
The problem is to find the minimum time required to reach the target cell in a grid, given the time required to move from one cell to another.
Identify the key concepts.
The key concepts involved in the problem are:
 The grid: This is a 2D array of cells, where each cell has a nonnegative integer value.
 The time required to move: This is the time required to move from one cell to another.
 The minimum time required to reach the target cell: This is the minimum time required to reach the target cell, starting from any cell in the grid.
Develop an approach.
One approach to solving the problem is to use dynamic programming. Dynamic programming is a technique for solving problems by breaking them down into smaller subproblems and using the solutions to the subproblems to solve the original problem.
In this problem, the subproblems are the paths that can be taken from the starting cell to the target cell. The solution to the subproblem of finding the minimum time required to reach a cell is the minimum of the minimum times required to reach the cell from each of its neighbors.
The overall solution can be implemented as follows:
 Initialize a table to store the minimum time required to reach each cell.
 Fill the table recursively, starting from the starting cell and working towards the target cell.
 The minimum time required to reach the target cell is the value stored in the table at the target cell.
Code in Python3:


Cues in the problem statement:
The cues in the problem statement that suggest the use of dynamic programming are:
 The problem can be broken down into smaller subproblems.
 The solutions to the subproblems can be reused.
 The problem has an optimal substructure.
Directions suggested in the approach to the problem:
The approach to the problem suggested by the cues in the problem statement is to use dynamic programming. This can be done by creating a table to store the minimum time required to reach each cell. The table can then be filled recursively, starting from the starting cell and working towards the target cell.
Insights about the problem statement:
The insights that can be gained from the problem statement are:
 The problem is a classic dynamic programming problem.
 The problem can be solved efficiently using dynamic programming.
 The problem can be solved in a recursive manner.
Bard failed after several attempts. Working solution from LC discussion:


The code first checks if the grid is invalid. If the grid is invalid, the code returns 1. Otherwise, the code initializes the following variables:
m
andn
: The dimensions of the grid.visited
: A set that keeps track of the cells that have already been visited.pq
: A priority queue that stores the cells that need to be visited.
The code then enters a loop that iterates over the priority queue. For each cell in the priority queue, the code does the following:
 Checks if the cell is the target cell. If the cell is the target cell, the code returns the time required to reach the cell.
 Checks if the cell has already been visited. If the cell has already been visited, the code skips the cell.
 Adds the cell to the set of visited cells.
 For each of the four neighbors of the cell, the code does the following:
 Checks if the neighbor is a valid cell. If the neighbor is not a valid cell, the code skips the neighbor.
 Checks if the neighbor has already been visited. If the neighbor has already been visited, the code skips the neighbor.
 Calculates the time required to reach the neighbor. The time required to reach the neighbor is the maximum of the time required to reach the current cell and the time required to reach the neighbor, plus the waiting time. The waiting time is 1 if the time required to reach the neighbor is even, and 0 if the time required to reach the neighbor is odd.
 Pushes the neighbor onto the priority queue.
The code continues looping until the priority queue is empty. If the priority queue is empty, the code returns 1.
The code is wellformatted and easy to read. The comments provide helpful explanations of the code. The code is also efficient and can be used to solve the problem for large grids.
From Brute Force to Optimal Solution
Here is a brute force solution for the problem:


The brute force solution works by simply doing a depthfirst search of the grid. The time complexity of the brute force solution is O(2^(m * n)), where m and n are the dimensions of the grid. The space complexity of the brute force solution is O(m * n), since the function needs to store the entire grid.
The brute force solution is inefficient because it does not take advantage of any of the structure of the problem. For example, the brute force solution does not consider the fact that the time required to reach a cell depends on the time required to reach its neighbors.
The following optimizations can be made to the brute force solution:
 Memoization: The brute force solution can be memoized to avoid recalculating the same paths multiple times. This will improve the time complexity of the solution to O(m * n * log(m * n)).
 Dynamic programming: The brute force solution can be implemented using dynamic programming. This will further improve the time complexity of the solution to O(m * n).
 Spatial partitioning: The brute force solution can be spatially partitioned to reduce the space complexity of the solution. This can be done by dividing the grid into smaller subgrids and then solving the problem for each subgrid independently.
The optimizations listed above will improve the time and space complexity of the solution. The specific improvements will depend on the specific implementation of the optimizations.
Here is an example of how the optimizations can impact the time and space complexity of the solution:
 Memoization: If the memoization table is implemented using a hash table, the space complexity of the solution will be O(m * n). The time complexity of the solution will be O(m * n * log(m * n)), since the function will need to search the memoization table for each path.
 Dynamic programming: If the dynamic programming table is implemented using an array, the space complexity of the solution will be O(m * n). The time complexity of the solution will be O(m * n), since the function will only need to iterate over the dynamic programming table once.
 Spatial partitioning: If the grid is partitioned into 4 subgrids, the space complexity of the solution will be O(m * n / 4). The time complexity of the solution will be O(m * n log(4)), since the function will need to solve the problem for each subgrid independently.
The specific optimizations that are used will depend on the specific problem and the specific constraints of the problem.
Code Explanation and Design Decisions
Here are the answers to your questions:
 The initial parameters are
grid
,m
, andn
.
grid
is a 2D array of cells, where each cell has a nonnegative integer value.m
is the number of rows in the grid.n
is the number of columns in the grid.
The significance of these parameters is that they define the problem domain. The grid represents the environment that the agent is moving through, and the values in the grid represent the time required to move from one cell to another. The number of rows and columns in the grid define the size of the environment.
 The primary loop is the while loop that iterates over the grid.
Each iteration of the loop represents one step that the agent takes. The loop starts at the topleft corner of the grid and moves to the right, then down, then to the left, and so on. At each step, the loop checks if the agent has reached the target cell. If the agent has reached the target cell, the loop terminates. Otherwise, the loop updates the agent’s position and time, and then iterates again.
 The conditions in the loop check if the agent has reached the target cell, and if the time required to reach the current cell is even or odd.
The condition that checks if the agent has reached the target cell is necessary because the loop only terminates when the agent has reached the target cell. The condition that checks if the time required to reach the current cell is even or odd is necessary because the waiting time that the agent needs to spend at a cell depends on whether the time required to reach the cell is even or odd.
 The parameters that are updated within the loop are the agent’s position and time.
The agent’s position is updated to the current cell. The agent’s time is updated to the maximum of the time required to reach the current cell and the time required to reach the neighbor, plus the waiting time.
The changes to the agent’s position and time reflect the changes in the state of the solution. The agent’s position changes as the agent moves through the grid, and the agent’s time changes as the agent spends time at cells.
 The invariant that is maintained throughout the code is that the agent is always moving towards the target cell.
This invariant is maintained because the loop only terminates when the agent has reached the target cell. The loop also ensures that the agent never moves backwards in the grid.
This invariant helps meet the problem’s constraints because it ensures that the agent will eventually reach the target cell.
 The final output is the time required to reach the target cell.
The significance of the final output is that it represents the solution to the problem. The time required to reach the target cell is the minimum amount of time that the agent needs to spend to reach the target cell.
This output satisfies the problem’s requirements because it is the minimum amount of time that the agent can spend to reach the target cell.
Coding Constructs
1. What are the highlevel problemsolving strategies or techniques being used by this code?
The highlevel problemsolving strategies or techniques being used by this code are depthfirst search and dynamic programming.
Depthfirst search is a recursive algorithm that explores a tree or graph by following each path as far as possible before backtracking. This algorithm is being used in this code to explore the grid of cells and find the minimum time required to reach the target cell.
Dynamic programming is a technique for solving problems by breaking them down into smaller subproblems and storing the solutions to the subproblems. This technique is being used in this code to store the minimum time required to reach each cell in the grid.
2. If you had to explain the purpose of this code to a nonprogrammer, what would you say?
The purpose of this code is to find the minimum time required to reach the target cell in a grid of cells. The grid of cells is represented by a 2D array, where each cell has a nonnegative integer value. The value in a cell represents the time required to move from the current cell to the neighboring cell.
3. Can you identify the logical elements or constructs used in this code, independent of any programming language?
The logical elements or constructs used in this code are conditional statements, loops, and recursion.
The conditional statements are used to check if the agent has reached the target cell, and if the time required to reach the current cell is even or odd. The loops are used to iterate over the grid of cells and to explore the subproblems. The recursion is used to call the dfs function recursively to explore the grid of cells.
4. Could you describe the algorithmic approach used by this code in plain English?
The algorithmic approach used by this code is as follows:
 Start at the topleft corner of the grid.
 Recursively explore the grid of cells, starting from the current cell.
 If the agent has reached the target cell, return the time required to reach the target cell.
 Otherwise, continue exploring the grid of cells.
 The algorithm terminates when the agent has reached the target cell or when all of the cells in the grid have been explored.
5. What are the key steps or operations this code is performing on the input data, and why?
The key steps or operations that this code is performing on the input data are:
 Initializing the variables m and n, which represent the number of rows and columns in the grid.
 Initializing the function dfs, which is used to explore the grid of cells.
 Calling the dfs function with the parameters (0, 0, 0), which represents the starting cell in the grid.
 Returning the value returned by the dfs function, which is the minimum time required to reach the target cell.
These steps are necessary to find the minimum time required to reach the target cell in the grid of cells.
6. Can you identify the algorithmic patterns or strategies used by this code, irrespective of the specific programming language syntax?
The algorithmic patterns or strategies used by this code are:
 Divide and conquer: The grid of cells is divided into smaller subproblems, and the minimum time required to reach each subproblem is found recursively.
 Memoization: The minimum time required to reach each subproblem is stored in a table, so that the time required to reach the subproblem does not need to be recalculated each time.
These patterns and strategies are used to improve the efficiency of the algorithm.
Language Agnostic Coding Drills
Here are the coding concepts or drills that I identified in the code:
 Initialization: The code initializes the variables
m
andn
, which represent the number of rows and columns in the grid. This is a basic concept that is common to many programming languages.  Function definition: The code defines the function
dfs
, which is used to explore the grid of cells. This is a more advanced concept that requires an understanding of functions and recursion.  Conditional statements: The code uses conditional statements to check if the agent has reached the target cell, and if the time required to reach the current cell is even or odd. This is a basic concept that is common to many programming languages.
 Loops: The code uses loops to iterate over the grid of cells and to explore the subproblems. This is a more advanced concept that requires an understanding of loops and recursion.
 Recursion: The code uses recursion to call the
dfs
function recursively to explore the grid of cells. This is a more advanced concept that requires an understanding of recursion.
I have listed the coding concepts in order of increasing difficulty, as follows:
 Initialization
 Function definition
 Conditional statements
 Loops
 Recursion
The difficulty of each concept depends on the programming language that is being used. For example, the concept of recursion is more difficult to implement in some programming languages than in others.
The problemsolving approach that would lead from the problem statement to the final solution is as follows:
 First, the problem statement must be understood. This involves understanding the meaning of the input data, the desired output, and the constraints on the solution.
 Once the problem statement is understood, the next step is to develop a solution strategy. This involves identifying the key steps that need to be taken to solve the problem, and the order in which the steps need to be taken.
 Once a solution strategy has been developed, the next step is to implement the solution in code. This involves translating the solution strategy into the specific syntax of the programming language that is being used.
 Finally, the code must be tested to ensure that it is correct. This involves running the code with a variety of input data and checking that the output is correct.
The coding drills listed above can be used to implement the solution to the problem. The difficulty of each drill will depend on the programming language that is being used. However, all of the drills are necessary to implement the solution correctly.
Targeted Drills in Python
Here are the Pythonbased coding drills for the concepts identified in the previous section:
Initialization


Function definition


Conditional statements


Loops


Recursion


Problemspecific concepts
In addition to the general concepts, the following problemspecific concepts are also needed to create a solution:
 Grid: A grid is a twodimensional array of cells. Each cell in the grid can be either empty or occupied.
 Agent: The agent is a moving object that can move from one cell to another. The agent’s goal is to reach the target cell.
 Target cell: The target cell is the cell that the agent is trying to reach.
Drills for problemspecific concepts


Integration of drills
The drills can be integrated together in the following order to solve the initial problem:
 Initialize the variables m and n.
 Create the grid.
 Initialize the agent at the starting cell.
 Iterate over the grid.
 If the agent is at the target cell, return.
 Otherwise, move the agent in a random direction.
 Repeat steps 46 until the agent reaches the target cell.
This is just one possible way to integrate the drills together. There are other ways to do it, but this is one way that is easy to understand and implement.
Q&A
Similar Problems
Here are 10 problems that require similar problemsolving strategies or use similar underlying concepts as the problem we’ve just solved:
 Minimum Path Sum
This problem is similar to the problem we just solved in that it involves finding the minimum cost to reach a target cell in a grid. The difference is that the cost of moving from one cell to another is not constant in this problem.
 Shortest Path in Binary Matrix
This problem is also similar to the problem we just solved in that it involves finding the shortest path to reach a target cell in a grid. The difference is that the grid in this problem is a binary matrix, meaning that each cell can either be 0 or 1.
 Unique Paths
This problem is similar to the problem we just solved in that it involves finding the number of possible paths to reach a target cell in a grid. The difference is that the paths in this problem cannot intersect.
 Robot in Grid
This problem is similar to the problem we just solved in that it involves moving an agent through a grid. The difference is that the agent in this problem can only move in four directions: up, down, left, and right.
 Flood Fill
This problem is similar to the problem we just solved in that it involves filling a grid with a certain color. The difference is that the grid in this problem is not rectangular.
 Maze Solver
This problem is similar to the problem we just solved in that it involves finding a path through a maze. The difference is that the maze in this problem can be more complex than the grid in the problem we just solved.
 Rat in Maze
This problem is similar to the problem we just solved in that it involves finding a path through a maze. The difference is that the rat in this problem can only move in four directions: up, down, left, and right.
 Knight’s Tour
This problem is similar to the problem we just solved in that it involves finding a path through a grid. The difference is that the knight in this problem can only move in an Lshaped pattern.
 Word Search
This problem is similar to the problem we just solved in that it involves finding a path through a grid. The difference is that the grid in this problem contains letters, and the goal is to find a path that spells out a certain word.
 Word Ladder
This problem is similar to the problem we just solved in that it involves finding a path between two words. The difference is that the path in this problem must consist of words that are related to each other.
These problems to be related to our original problem because they all involve finding a path through a grid or maze. The differences between the problems are the specific rules that govern how the path can be found.