Graph Connectivity With Threshold
To solve this problem, we can apply the UnionFind algorithm, also known as the DisjointSet data structure. This algorithm will help us to connect the cities that share a common divisor strictly greater than the threshold and then check if the cities in the queries are connected or not.
Here are the main steps:
Initialization: Create a parent array to keep track of the root of each city.
Union: Connect cities that share a common divisor strictly greater than the threshold. For each number (i) from the threshold + 1 to (n), connect all the multiples of (i) as they will share (i) as a common divisor.
Find: Define a function to find the root of a city. This will help in checking if two cities are connected.
Answer Queries: For each query, check if the roots of the cities are the same. If yes, they are connected; otherwise, they are not.
Here’s the code:


This code efficiently determines if the cities in the queries are connected based on the given conditions, using the UnionFind algorithm.
Identifying Problem Isomorphism
“Graph Connectivity With Threshold” is a graph theory problem where you have to determine whether queries about node connectivity in a graph are true or not, given a certain threshold for edge weights.
A simpler problem which shares some aspects of this problem is “Find the City With the Smallest Number of Neighbors at a Threshold Distance”. Here, the task is to find the city that has the smallest number of reachable cities within or equal to distance threshold.
An approximate isomorphic problem is “Network Delay Time”. This problem, like “Graph Connectivity With Threshold”, involves graph traversal and applying conditions based on edge weights. However, “Network Delay Time” has additional complexity as it includes finding the minimum time it will take for all nodes to receive a signal.
A more complex problem is “Path With Minimum Effort”. This problem involves finding the path in a 2D grid from topleft to bottomright that minimizes the maximum absolute difference in heights between two consecutive cells in the path, which is a higher level of complexity than the original problem.
Hence, from simplest to more complex:
 “Find the City With the Smallest Number of Neighbors at a Threshold Distance”  Determine the city with the least number of reachable cities within or equal to distance threshold.
 “Graph Connectivity With Threshold”  Determine whether queries about node connectivity in a graph are true, given a certain threshold for edge weights.
 “Network Delay Time”  Find out how long it will take for a signal to reach all nodes in the network, given the transmission time between nodes.
 “Path With Minimum Effort”  Determine the path with minimum effort in a 2D grid.
10 Prerequisite LeetCode Problems
“1627. Graph Connectivity With Threshold” involves concepts such as number theory, unionfind data structure, and graph theory. To solve this problem, you need to understand how to build a graph and how to perform efficient queries regarding its connected components.
Here are some simpler problems to build up to it:
Number of Islands (LeetCode #200): This problem involves finding connected components in a grid, which is an easier version of the problem scenario.
Friend Circles (LeetCode #547): This problem requires finding connected components in a graph represented as an adjacency matrix, similar to finding the connected nodes in the original problem.
Redundant Connection (LeetCode #684): This problem introduces the concept of unionfind, which is crucial for the original problem.
Connecting Cities With Minimum Cost (LeetCode #1135): This problem involves finding the minimum cost to connect all nodes in a graph, a concept which is extended in the original problem.
Most Stones Removed with Same Row or Column (LeetCode #947): This problem involves removing stones based on the connectivity of the graph, a similar concept to the original problem.
Smallest String With Swaps (LeetCode #1202): This problem involves swapping elements based on the connectivity of the graph, a concept extended in the original problem.
Accounts Merge (LeetCode #721): This problem involves merging accounts based on email connectivity, a similar problem setup to the original problem.
Path With Maximum Minimum Value (LeetCode #1102): This problem involves finding the maximum minimum value path in a graph, which can be seen as an extension of the problem scenario.
Number of Operations to Make Network Connected (LeetCode #1319): This problem involves finding the minimum number of operations to connect all computers in a network, a concept which is also found in the original problem.
Satisfiability of Equality Equations (LeetCode #990): This problem involves solving a system of equations based on equality and inequality, which is a similar problem setup to the original problem.
These cover unionfind data structure, as well as graph theory and number theory concepts, which are crucial for solving the original problem.
Problem Analysis and Key Insights
DivisorBased Connectivity: The cities are connected based on a mathematical relationship—having a common divisor greater than a given threshold. This is a departure from typical graph problems where edges are explicitly provided.
Threshold Importance: The threshold plays a crucial role. It can dramatically alter the connectivity between cities. A lower threshold makes more connections possible, while a higher one limits the connectivity.
Multiple Queries: You’re expected to answer multiple queries efficiently. This suggests that precomputation might be beneficial, especially given that you are to run up to 10^5 queries.
Bidirectional Roads: All roads are bidirectional, so connectivity is not directed. If city A is connected to city B, then B is also connected to A.
Graph Structure: The graph is not given; you must construct it yourself based on the divisor and threshold conditions. This means you will likely need to handle both the divisor logic and the graph traversal logic.
Indirect Connectivity: The problem asks for both direct and indirect connectivity, suggesting that classic graph algorithms like UnionFind or BFS/DFS might be useful for the traversal part of the problem.
Constraints: The constraints in terms of n, threshold, and the number of queries give an idea about the expected time complexity. Since n can go up to 10^4 and the number of queries up to 10^5, the solution should be better than O(n^2) to be efficient.
Equivalent Queries: The problem statement specifies that the queries are symmetrical ([x, y] is the same as [y, x]). This implies that we could use this symmetry to reduce computational time.
These insights point towards the need for an efficient algorithm that can handle both the number theory aspects (finding divisors) and the graph theory aspects (checking connectivity) in a cohesive manner.
Problem Boundary
The scope of the problem spans multiple domains:
Number Theory: You have to understand divisors and how to efficiently find them for each city to establish direct connections.
Graph Theory: Once connections between cities are established, the problem converts into a graph problem where you need to find if there is a path between any two given cities, either directly or indirectly.
Query Optimization: Given that there can be up to 10^5 queries, an efficient mechanism for answering these is crucial.
Data Structures: Efficient data structures might be required for storing the graph and quickly answering queries. This could involve structures like arrays, sets, or disjointset data structures.
Algorithmic Complexity: Given the constraints, the solution’s time and space complexity will need to be carefully considered to ensure it runs within acceptable limits.
The problem, therefore, isn’t just about solving a single question but about integrating multiple computational concepts effectively.
The boundaries of this problem are defined by the constraints and requirements outlined in the problem statement:
Input Size:
 ( n ) is between 2 and ( 10^4 )
 Threshold is between 0 and ( n )
 Number of queries is between 1 and ( 10^5 )
Data Types:
 All inputs are integers.
Operational Constraints:
 Divisibility conditions to establish connections between cities.
 Each query involves a pair of distinct cities.
Output:
 An array of Boolean values corresponding to each query, indicating if a path exists between the cities in that query.
Performance:
 The solution must efficiently handle up to ( 10^4 ) cities and ( 10^5 ) queries.
Functional Requirements:
 Handle multiple queries.
 The query ([x, y]) is equivalent to ([y, x]).
By clearly understanding these constraints and requirements, you set the boundaries within which the solution must operate. This helps in selecting appropriate algorithms and data structures to solve the problem effectively.
Problem Classification
The problem falls under the Graph Theory and Number Theory domain. It combines elements of finding divisors and graph connectivity.
‘What’ Components:
We have
n
cities labeled from 1 ton
.Cities
x
andy
are connected if they have a common divisorz > threshold
.Given
n
, athreshold
, and an arrayqueries
, we must find if the cities in each query are directly or indirectly connected.Connectivity Problem: We need to check if there exists a path between two nodes (cities), either directly or indirectly.
Divisibility Constraints: The problem requires us to find divisors to establish connections between nodes.
Multiple Queries: Multiple scenarios need to be checked for connectivity.
Connectivity Problem: We are interested in finding out if there’s a path between two given cities. This is a classic graph problem where nodes represent cities and edges represent roads between them.
Divisibility Constraints: What makes it unique is how the connections are determined. The edges are not provided; instead, we have to establish them based on the criteria involving divisors and thresholds.
Multiple Queries: The problem asks us to check for multiple pairs, making it imperative that the solution is efficient enough to handle multiple cases in a reasonable time.
Distilling the Problem to Its Core Elements
Fundamental Concept:
 The problem is fundamentally based on graph theory and number theory. We need to establish connections between nodes (cities) based on common divisors and then query for connectivity.
Simplest Description:
 Imagine a network of cities connected by roads. Two cities are directly connected only if they share a common number greater than a certain limit. You’re given a list of city pairs, and your job is to find out if you can travel from one city to another in each pair, using these roads.
Core Problem:
 Determine if there’s a path between two given cities, either directly or through other cities, based on the stated divisibility rules.
Key Components:
 Identify common divisors between each pair of cities.
 Build connections (edges) between cities based on common divisors.
 Query connectivity between pairs of cities in the list.
Minimal Set of Operations:
 Compute divisors for each city.
 Establish city connections.
 Perform connectivity queries for each city pair in the list.
By breaking the problem down this way, you gain a clearer understanding of what you need to solve. This helps in choosing the right algorithms and data structures.
Visual Model of the Problem
Cities as Nodes:
 Imagine each city as a node in a graph.
Divisors as Conditions for Edges:
 An edge (road) exists between two nodes if they share a common divisor greater than the threshold.
Queries as Paths:
 Each query can be visualized as checking for a path between two specific nodes in the graph.
Threshold as a Filter:
 Imagine a filter that only allows certain edges to be formed based on the divisor condition.
So, to visualize:
 Start with
n
disconnected nodes.  Compute divisors for each pair of nodes.
 Add edges between nodes that meet the divisor conditions.
 For each query, visualize checking for a path between the two nodes in question.
By this visualization, you convert an abstract problem with numbers and divisors into a more concrete problem involving graphs and paths, which is often easier to conceptualize and solve.
Problem Restatement
You have a range of cities numbered from 1 to n. A road connects two cities if they have a common divisor greater than a given threshold. Your task is to answer a series of queries. Each query asks whether there is a direct or indirect path between two specific cities. Return an array of boolean values corresponding to the queries: ’true’ if a path exists, ‘false’ otherwise.
Requirements:
 n is the number of cities, and it ranges from 2 to 10^4.
 The threshold is a nonnegative integer up to n.
 Each query is an array of two integers, representing the cities to check for a connection.
 The cities in each query are different.
Constraints:
 The array of queries can have up to 10^5 elements.
The goal is to find out which pairs of cities are connected based on these rules.
Abstract Representation of the Problem
In the abstract, you have a set of nodes labeled from 1 to n. Edges between these nodes exist based on a function of their labels and a given constant. The function is based on finding common divisors greater than this constant. You have to answer a series of queries asking whether there exists a path between pairs of nodes. A path may be direct or through other intermediate nodes. Return an array of boolean values, each corresponding to a query, indicating whether a path exists or not.
Key elements:
 Set of nodes (1 to n)
 Function to establish edges based on node labels and a given constant
 Array of queries asking for existence of a path between node pairs
Structure:
 Nodes and Edges (Graph)
 Function (Determines edges)
 Queries (Path existence between nodes)
Terminology
Bidirectional Road (Edge): A connection between two cities (nodes) that allows travel in both directions. It’s crucial for defining the relationships between cities.
Threshold: A constant that sets a minimum limit on the common divisor needed to form an edge. It helps in controlling the density and nature of connections between nodes.
Common Divisor: A number that evenly divides two other numbers. It serves as the basis for establishing an edge in this problem.
Queries: Pairs of node labels for which we need to find if a path exists. These form the practical questions we’re trying to answer.
Path: A sequence of nodes connected by edges. In this context, we’re interested in whether a path exists between two given nodes, either directly or indirectly.
Graph: A collection of nodes and edges. Here, it serves as the abstract representation of cities and their connections.
Boolean Array: An array of True/False values. It is used to represent the answers to the queries.
Understanding these terms and concepts is crucial for both formulating and solving the problem. They define the rules and objectives, giving structure to the problem space.
Problem Simplification and Explanation
Let’s break it down:
 Cities: Think of these as points on a piece of paper.
 Roads: These are lines connecting the points (cities). However, we can’t draw lines randomly; they have to meet certain conditions based on divisors and thresholds.
 Threshold: It’s like a filter or a minimum requirement for connecting cities. Only if two cities have a common number (divisor) bigger than this threshold, we can connect them with a road.
 Queries: These are questions asking whether you can go from one city to another, following the roads.
Interaction: You first decide which cities (points) get connected based on their common divisors and the threshold (filter). Once you have all the roads drawn, you answer the queries by checking if you can travel from one city to another using these roads.
Analogy: Imagine a class of students, and each student has a set of skills (like divisors). Now, they can only become friends (get connected) if they share a skill that is above a certain level of coolness (the threshold). Once friendships are formed, queries are like asking, “Can you introduce Student A to Student B through a chain of friends?”
This breaks down the complexities into manageable parts: figuring out the rules for friendships (roads), then answering questions about the social network that forms.
Constraints
Numerical Range: The number of cities ( n ) is constrained to be ( \leq 10^4 ), and the number of queries ( \leq 10^5 ). These constraints hint at the feasibility of a solution that runs in ( O(n \log n) ) or ( O(n \sqrt{n}) ) time complexity.
Threshold: The threshold can never be greater than ( n ). If it’s zero or close to zero, almost every city will be connected to every other city, simplifying the problem.
Divisors: The problem revolves around common divisors greater than the threshold. Notice that if ( x % z == 0 ) and ( y % z == 0 ), ( z ) itself should be less than both ( x ) and ( y ). This helps to limit the search space when checking for divisors.
Connectedness: The connections are transitive. If city ( A ) is connected to ( B ), and ( B ) is connected to ( C ), then ( A ) is connected to ( C ). This suggests that we can use a data structure to keep track of connected components, like UnionFind or DFS.
Multiple Queries: Since there are multiple queries, any precomputation that helps answer each query faster can be very beneficial.
Query Pairs: ( a_i ) and ( b_i ) in queries are not the same (( a_i \neq b_i )). This removes the need to handle identical pairs.
By observing these characteristics, we can start thinking of strategies to solve the problem more efficiently.
Bounded Input Size: The constraints on ( n ) and the number of queries suggest that a solution with a time complexity of ( O(n \log n) ) or ( O(n \sqrt{n}) ) could be feasible. Algorithms with higher time complexity are likely to be inefficient.
Threshold Behavior: If the threshold is zero, every city will be connected to every other city, making the problem simpler. The nonnegative constraint on the threshold indicates that using the threshold effectively can drastically reduce the complexity of the problem.
Nonidentical Query Pairs: The constraint that ( a_i \neq b_i ) in each query simplifies the problem by eliminating the need to consider selfconnections.
Transitive Connections: The problem asks for direct or indirect connections, hinting at the possibility of using data structures that can manage such relationships efficiently, like UnionFind or DFS trees.
Repetitive Nature of Queries: The high number of queries compared to ( n ) suggests that optimizing for query speed could be beneficial, even if it requires a higher initial computational cost.
Integer Divisors: Since we are dealing with integers and divisors, mathematical properties around divisibility can potentially be exploited for optimization.
These insights from the constraints can guide us in selecting appropriate algorithms and data structures for an efficient solution.
Case Analysis
Additional Examples and Test Cases
Case 1: Minimal Input Values (“Smallest Set”)
 Input: ( n = 2 ), ( \text{threshold} = 0 ), ( \text{queries} = [[1,2]] )
 Output: [true]
 Analysis: The smallest possible value for ( n ) and ( \text{threshold} ). Here, since the threshold is zero, any two cities are connected. This example tests if the program can handle the minimum constraints.
Case 2: No Shared Divisors (“Divisorless Pair”)
 Input: ( n = 6 ), ( \text{threshold} = 3 ), ( \text{queries} = [[4,5]] )
 Output: [false]
 Analysis: Here, 4 and 5 don’t share a common divisor greater than 3. This tests if the program correctly identifies cities that aren’t connected due to the divisor condition.
Case 3: All Cities Connected (“Zero Threshold”)
 Input: ( n = 4 ), ( \text{threshold} = 0 ), ( \text{queries} = [[1,3], [2,4], [1,4]] )
 Output: [true, true, true]
 Analysis: When the threshold is zero, all cities are connected. This example tests if the program accounts for this special case.
Case 4: Single Query, Single Connection (“One Pair”)
 Input: ( n = 5 ), ( \text{threshold} = 2 ), ( \text{queries} = [[2, 4]] )
 Output: [true]
 Analysis: This case aims to check whether the algorithm correctly identifies that 2 and 4 are the only pair connected directly, having 2 as a shared divisor which is greater than the threshold.
Case 5: Multiple Queries, Varied Connections (“Mixed Bag”)
 Input: ( n = 7 ), ( \text{threshold} = 1 ), ( \text{queries} = [[3, 6], [4, 5], [1, 7]] )
 Output: [true, false, false]
 Analysis: Here, only 3 and 6 share a common divisor (3) greater than the threshold. This tests if the program can handle multiple queries with varied outcomes.
Case 6: All Cities Disconnected (“High Threshold”)
 Input: ( n = 4 ), ( \text{threshold} = 4 ), ( \text{queries} = [[1, 2], [2, 3], [3, 4]] )
 Output: [false, false, false]
 Analysis: Here, the threshold is too high for any cities to be connected. This tests if the program correctly identifies such scenarios.
Edge Cases
 Smallest Set: The smallest possible input values for ( n ) and ( \text{threshold} ).
 Zero Threshold: A zero threshold means all cities should be connected.
 High Threshold: A threshold so high that no cities are connected.
 Mixed Bag: Multiple queries with different expected outcomes, ensuring that the program can handle complexity and variety.
These test cases aim to cover a broad input space, testing both the normal operational conditions and the boundaries of the problem.
Smallest Set: The algorithm should be able to handle the minimum constraints efficiently.
Divisorless Pair: The algorithm needs to accurately determine when there’s no common divisor greater than the threshold, implying that the cities aren’t connected.
Zero Threshold: If the threshold is zero, all cities are connected, and the algorithm should be optimized for this quick determination.
One Pair: The program should be precise enough to identify when only one or a specific subset of city pairs are connected.
Mixed Bag: The algorithm must be able to handle multiple queries effectively and return varied results based on the input.
High Threshold: The algorithm should recognize when no cities are connected due to a high threshold, returning all
false
as needed.
Key Insights:
Threshold Sensitivity: The threshold plays a critical role in determining the connections. When it’s zero, all cities connect; when it’s too high, none connect.
Multiple Queries: The algorithm will often need to process multiple queries, so it should be designed to do this efficiently, perhaps reusing calculations.
Special Cases: Cases like all cities being connected or none being connected can likely be solved very quickly without extensive calculations, so the algorithm should identify these early on.
Optimization: The wide range of potential inputs suggests that the algorithm will need to be optimized for both small and large inputs. The
n
value and the length of the queries array can vary greatly.
By analyzing these different cases, we can see that the algorithm must be versatile and optimized to handle a range of scenarios effectively.
Identification of Applicable Theoretical Concepts
Graph Theory: The problem essentially boils down to finding connected components in a graph where each node represents a city, and each edge signifies a connection based on the divisor condition. This can be efficiently handled using UnionFind (Disjoint Set Union) algorithms.
Number Theory: The problem requires finding common divisors greater than a given threshold for pairs of numbers. Insights from number theory, particularly around factors and multiples, can be helpful.
Dynamic Programming/Memoization: Since multiple queries can ask about the same or overlapping sets of cities, storing previously calculated results can eliminate redundant calculations and speed up the algorithm.
Complexity Analysis: BigO notation and algorithmic complexity could guide us in optimizing our solution for different scales of problem sizes, considering the worstcase and averagecase scenarios.
Shortcircuiting: Certain cases, like a zero threshold, can allow for an immediate answer, avoiding the need for further calculations.
BreadthFirst Search (BFS) or DepthFirst Search (DFS): These could also be used for finding connected components, though they might be less efficient than UnionFind for this particular problem.
Batch Processing: Given that we have multiple queries, it might be more efficient to process them in batches if it allows us to reuse some computed results.
By applying these mathematical and algorithmic concepts, we can simplify the problem and find an efficient solution. Each concept contributes either to the speed, correctness, or clarity of the final algorithm.
Simple Explanation
Imagine you have a bunch of cities, and some cities are connected by roads. Now, these roads aren’t built randomly. They follow a special rule. Each city has a number, and two cities are connected if their numbers share a common “friend” that’s greater than a certain value. Think of this “friend” like a common hobby two people must share to become friends, except here it’s a number both city numbers can be divided by.
So if City 1 has the number 12 and City 2 has the number 16, they both like the number 4 (because 12 and 16 can be divided by 4). If the value we’re looking for is 3 or less, then a road can be built between these cities.
Now, someone gives you a list of pairs of cities and asks, “Can you travel between these two cities by road, either directly or by passing through other cities?” You have to figure this out for each pair on the list.
In everyday terms, it’s like a game of “Six Degrees of Kevin Bacon,” but instead of finding connections between people through movies they’ve been in, you’re finding connections between cities based on these “friend numbers.”
Problem Breakdown and Solution Methodology
To solve this problem, let’s take a layered approach. Here’s how I’d break it down:
Identify Common Divisors: The first thing we need is to figure out which cities are connected. For that, we’ll look at every pair of cities and find if they have a common divisor greater than the threshold. Think of this step as creating a Facebook friend list for each city, where ‘friends’ are other cities that share a common divisor.
Building the Road Map: After we know which cities are directly connected, we build a ‘road map’. In computer science, this is known as a graph. Each city is a ’node’, and if two cities are directly connected, there is an ’edge’ or a line between those nodes. This step is like drawing lines between Facebook friends to visually represent who’s connected to whom.
Path Finding: Now that we have our ‘road map’, for each query, we have to find if there is a path between the two cities. This could be a direct path or a path that goes through one or more other cities. It’s like trying to find if you can introduce one friend to another through a chain of mutual friends.
Answering Queries: Finally, for each query, we’ll return ’true’ if a path exists between the cities in question and ‘false’ otherwise.
Effects of Parameter Changes:
Changing n (Number of Cities): The more cities you have, the more pairs you’ll have to check, which would make the solution slower.
Changing Threshold: A higher threshold means fewer direct connections, making the graph sparser. A lower threshold would result in a denser graph, potentially making the pathfinding step more complicated but also more likely.
Changing Number of Queries: More queries mean more time spent in the pathfinding step.
Example Case:
Let’s say we have 4 cities with the threshold as 2. The numbers are 1, 2, 3, and 4. The divisors would be:
 City 1: [1]
 City 2: [1, 2]
 City 3: [1, 3]
 City 4: [1, 2, 4]
Common Divisors:
 Cities 2 and 4 share a common divisor, 2, which is greater than the threshold. So they are directly connected.
Building Road Map:
 An edge between City 2 and City 4.
Path Finding:
 Query 1: [1, 2]  No path
 Query 2: [2, 4]  Direct path exists
 Query 3: [3, 4]  No path
Answering Queries:
 [false, true, false]
So that’s how I’d approach solving this problem. By breaking it down into these layers, you can focus on each part separately before putting it all together for the solution.
Inference of ProblemSolving Approach from the Problem Statement
Here are the key terms and how they guide the problemsolving approach:
Cities: These are the nodes in our graph. Knowing that cities are represented by numbers allows us to use an array or a list for efficient access.
Threshold: This informs our divisor checking. Only common divisors greater than the threshold count for making connections. It’s a filter that reduces the number of edges in our graph.
Divisor: Understanding that connections are based on divisors informs us that we’ll need to do some mathheavy operations. It’s crucial for the “Identify Common Divisors” step.
Bidirectional Road: This indicates that if city
A
is connected to cityB
, thenB
is also connected toA
. This tells us that the graph is undirected.Directly or Indirectly Connected: This phrase suggests that we might have to find paths that are not just direct connections but sequences of connections. This immediately points toward using graph traversal algorithms like DFS (DepthFirst Search) or BFS (BreadthFirst Search).
Queries: This is the list of city pairs we need to check for a connection. Knowing that there could be multiple queries informs us that we might benefit from precomputing as much information as possible before answering the queries.
Array Answer: Knowing that the output is an array means that we need to keep track of our answers in a listlike data structure.
Constraints: These give us an idea of the input size and help us choose appropriate data structures and algorithms that will run in a reasonable time.
Understanding these key terms and concepts narrows down the methods and algorithms suitable for solving the problem. For example, understanding that we need to find “Directly or Indirectly Connected” cities leans toward graph traversal algorithms. The term “Threshold” hints at a filtering step before creating the graph, and so on.
How did you infer from the problem statement that this problem can be solved using ?
Simple Explanation of the Proof
I’m having trouble understanding the proof of this algorithm. Could you explain it in a way that’s easy to understand?
Stepwise Refinement
Initialize Data Structures:
 Create an empty graph data structure to represent the cities and their connections.
Calculate Divisors and Populate Graph:
 For each pair of cities
(A, B)
, calculate their divisors.  If they share a divisor greater than the threshold, add an edge between them in the graph.
 For each pair of cities
Graph Traversal Preparation:
 Initialize data structures for graph traversal, such as a visited set and a queue for BFS.
Query Handling:
 For each query
(Ai, Bi)
, use BFS or DFS to check ifAi
andBi
are connected directly or indirectly in the graph.  Store the result for this query in an output list.
 For each query
Finalize and Return the Output:
 Convert the output list to the required format (an array) and return it.
Granular, Actionable Steps
Initialize Data Structures:
 Use a dictionary or an adjacency list to represent the graph.
Calculate Divisors and Populate Graph:
 Iterate from city 1 to city N.
 In a nested loop, iterate from the current city to city N.
 Calculate divisors and check if they are greater than the threshold.
 If yes, add an edge in the graph’s adjacency list.
Graph Traversal Preparation:
 Create an empty set for visited nodes.
 Create an empty queue for BFS.
Query Handling:
 For each query, initialize the BFS queue with the starting city
Ai
.  Mark
Ai
as visited.  While the queue is not empty, remove a city and check its neighbors.
 If
Bi
is found, break and mark this query asTrue
in the output list.  If the queue empties without finding
Bi
, mark this query asFalse
.
 For each query, initialize the BFS queue with the starting city
Finalize and Return the Output:
 Return the output list as an array.
Parts That Can Be Solved Independently
 The divisor calculation and graph population can be done independently before handling any queries.
 Each query can be processed independently after the graph is created.
Repeatable Patterns
 The calculation of divisors for different city pairs is a repeatable pattern.
 Graph traversal (BFS or DFS) for each query is a repeated operation.
 Populating the output list is a repeatable pattern done after each query.
By breaking down the problem in this way, you can work on each component separately, test it, and then integrate it into the final solution.
Solution Approach and Analysis
Step 1: Initialize Data Structures
First, you’ll need a way to represent the connections between cities. Think of this as a social network, where each city is a person and connections between them are friendships. You can use an adjacency list for this.
Step 2: Calculate Divisors and Populate Graph
For every pair of cities, you’ll find common divisors that are greater than the threshold. If you find such a divisor, connect the two cities in your “social network”. It’s like saying if two people have a common interest above a certain level, they become friends.
Step 3: Prepare for Graph Traversal
Before you start answering queries, prepare for a way to traverse your social network. You could use BreadthFirst Search (BFS). For this, initialize an empty ‘Visited’ list and a ‘Queue’.
Step 4: Query Handling
For each query, you’ll start at one city and see if you can reach the other through any path. This is similar to checking if one person can be reached through a chain of friendships starting from another person.
Step 5: Finalize and Return the Output
Once you’ve answered all queries, prepare your answer list for output.
How Parameters Affect the Solution
 Number of Cities (n): Increasing this would make the graph larger, increasing computation time.
 Threshold: A higher threshold would mean fewer connections, making the graph sparser and easier to traverse.
 Number of Queries: More queries would mean more graph traversals, increasing computation time.
Workings with Example Cases
Example with n=6, threshold=2, and queries=[[1,4],[2,5],[3,6]]
 Step 1: Initialize an empty adjacency list.
 Step 2: Connect only 3 and 6 as they share a common divisor 3, which is > 2.
 Step 3: Prepare ‘Visited’ and ‘Queue’ for BFS.
 Step 4: Query [1,4] cannot be connected, same for [2,5]. But [3,6] are directly connected. So, output is
[false, false, true]
.
Example with n=6, threshold=0, and queries=[[4,5],[3,4],[3,2],[2,6],[1,3]]
 Step 13: Similar to the first example.
 Step 4: Here, since the threshold is 0, all cities are connected. So, output is
[true, true, true, true, true]
.
By breaking down the problem like this, you’re ensuring that you’re tackling each part systematically and not missing any important details.
Identify Invariant
The invariant in this problem is the condition for connecting two cities. No matter what other parameters you change or how many queries you process, the fundamental rule remains the same: two cities ( x ) and ( y ) are directly connected if there exists an integer ( z ) such that ( x \mod z == 0 ), ( y \mod z == 0 ), and ( z > \text{threshold} ).
This invariant is crucial because it guides the entire solution process. It dictates how you populate your graph, which in turn determines how you answer each query. It remains consistently true throughout the problem.
Identify Loop Invariant
In the context of this problem, a loop invariant would be a condition or property that holds true before and after each iteration of a loop used in your algorithm. While the problem statement itself doesn’t specify the use of loops, solving this problem will likely involve loops for constructing the connections between cities and for evaluating the queries.
A potential loop invariant, if you use a loop to iterate through pairs of cities to check for connections, might be: “For each pair of cities (x) and (y) that the loop has checked so far, their connection status (either connected or not connected) has been correctly determined and stored.”
This loop invariant ensures that the fundamental rules for connecting cities have been correctly applied to all pairs so far, allowing you to proceed with confidence in the subsequent steps of your algorithm.
In this problem, both the invariant and loop invariant could serve the same purpose of ensuring that the connectivity between cities is correctly determined as per the rules.
The invariant for the entire algorithm might state: “For any pair of cities (a) and (b) that the algorithm has processed, the connectivity status is correctly identified.”
If you’re using a loopbased approach, like iterating through all pairs to find connections, the loop invariant would be similar but confined to the loop’s scope: “For every iteration of the loop, the algorithm correctly determines the connectivity for the pairs it has processed so far.”
So, in essence, they both aim to validate the same property but at different scopes within the algorithm. The loop invariant applies to each loop iteration, while the overall invariant applies to the algorithm as a whole.
If you opt for a recursive approach, such as depthfirst search (DFS) or unionfind with path compression, an invariant during recursion could be: “For any given pair of cities (a) and (b) that have been processed through the recursion so far, the connectivity status is correctly determined.”
This invariant ensures that as you navigate recursively through connected cities, you maintain the correct status of whether or not there is a path between the cities being checked. If you’re using a recursive function to traverse from one city to another, the invariant confirms that the function will correctly find a path if one exists, according to the rules for connectivity outlined in the problem statement.
Thought Process
Basic Thought Process and Steps
Understand the Problem: The first step is understanding the problem clearly. We are given ( n ) cities, a threshold, and queries asking if there exists a path between two cities.
Identify Constraints and Conditions: The problem states that a path exists between two cities ( x ) and ( y ) if there exists a common divisor ( z > ) threshold. The problem statement’s constraints and conditions hint that you could use graph algorithms and unionfind data structures.
Formulate a Plan: Create a data structure to maintain city connections and then iterate through pairs of cities to determine if they are connected based on the given rules. Afterwards, answer the queries based on this structure.
Divide and Conquer: Break the problem into subproblems:
 Subproblem 1: Finding divisors and applying the threshold condition
 Subproblem 2: Storing the connectivity info
 Subproblem 3: Answering queries based on the stored info
Code the Solution: Write the code, paying attention to efficiency.
Testing: Use a range of test cases including edge cases to validate the solution.
Code
Here’s how you could write this in Python3:


Cues and Insights from Problem Statement
 Threshold Condition: The threshold value can eliminate trivial connections (like 1 being a divisor for all). This speeds up computation.
 Direct or Indirect Connection: This hints at a need for a unionfind data structure or a graph traversal algorithm to identify connected components.
This stepbystep solution uses unionfind to manage connections efficiently, based on the cues from the problem statement.
Establishing Preconditions and Postconditions
Parameters:
Inputs:
n
(integer): Represents the number of cities.threshold
(integer): The divisor threshold for connecting cities.queries
(list of pairs): List of pairs representing the cities to check for connectivity.
Types:
n
: integerthreshold
: integerqueries
: list of pairs of integers
Context:
n
tells us the total number of cities.threshold
gives us the minimum divisor needed for a connection.queries
lists the specific city pairs whose connectivity we are interested in.
Preconditions:
State:
 None. The method does not depend on any global state.
Constraints:
 1 ≤
n
≤ 10^4  0 ≤
threshold
<n
queries
will have at most 10^4 elements, and each query will have two integers between 1 andn
.
 1 ≤
Program State:
 None.
Method Functionality:
Purpose:
 The method should determine if there exists a path between pairs of cities based on the given divisor threshold.
Interaction:
 It uses the inputs to build connections between cities and answers queries about those connections.
Postconditions:
State:
 The program state remains unchanged, as the function does not have any side effects.
Return Value:
 A list of boolean values representing whether a path exists for each query.
Side Effects:
 None.
Error Handling:
Preconditions Not Met:
 If the inputs do not meet the stated constraints, the behavior is undefined as per the problem statement.
Exception Handling:
 The method itself doesn’t include explicit error handling for invalid inputs. It assumes that inputs adhere to the described constraints.
Problem Decomposition
Problem Understanding:
In My Own Words:
 The problem involves connecting cities based on divisors of their respective numbers. Given pairs of cities, the goal is to find out if these pairs are connected through a path, directly or indirectly, based on a divisor threshold.
Key Components:
 Number of cities (
n
), Divisor threshold (threshold
), Pairs of cities (queries
).
 Number of cities (
Initial Breakdown:
 Major Parts:
 Initialize data structures for city connections.
 Create connections based on the divisor threshold.
 Answer connectivity queries for given pairs of cities.
Subproblem Refinement:
Data Structures Initialization:
 Initialize UnionFind data structure to keep track of city groups.
Creating Connections:
 Loop through city pairs and connect them if they have a common divisor greater than the threshold.
Answering Queries:
 For each query, check if the pair of cities belong to the same group in the UnionFind data structure.
Task Identification:
 Repeated Tasks:
 The task of checking whether two numbers have a common divisor greater than the threshold can be repeated for multiple pairs.
Task Abstraction:
Check Divisors:
 Create a function to check if two numbers have a common divisor greater than the threshold.
UnionFind Operations:
 Abstract Union, Find operations into functions.
Answer Query:
 Abstract the task of answering a single query into a function.
Method Naming:
Check Divisors:
hasCommonDivisorAboveThreshold
UnionFind Operations:
unionCities
,findCityGroup
Answer Query:
isConnected
Subproblem Interactions:
Interaction:
 Data structures must be initialized before creating connections.
Order:
 Initialize data structures.
 Create connections.
 Answer queries.
Dependencies:
 Query answering depends on the connections made earlier.
From Brute Force to Optimal Solution
Brute Force Solution:
Initialize a 2D array (
adjMatrix
) of sizen x n
toFalse
. This will represent if there’s a direct path between two cities.adjMatrix[i][j]
will beTrue
if there’s a path from cityi
to cityj
.Populate the 2D array: Loop through every pair of cities
(i, j)
and setadjMatrix[i][j] = True
ifi
andj
have a common divisor greater than the threshold.Answer Queries: For each query pair
(a, b)
, loop through every possible intermediate cityc
to check if you can get froma
tob
throughc
.
Inefficiencies:
Time Complexity: The time complexity for populating the 2D array is
O(n^2)
. For each query, checking all paths takes up toO(n^2)
. So, forq
queries, it becomesO(q * n^2)
.Space Complexity: The 2D array takes
O(n^2)
space.
Optimization Steps:
Use UnionFind Data Structure: Instead of using a 2D array to keep track of connections, use a UnionFind data structure. This will allow us to quickly find if two cities are connected in almost
O(1)
time.Batch Processing for Connections: Instead of looping through every pair to find divisors, sort the numbers and use the divisors of smaller numbers to find connections in one go.
Optimized Approach:
Initialize UnionFind Data Structure: Each city is its own group initially.
Create Connections Efficiently: Sort the cities by their numbers. For each city
i
, find its divisorsd
and check ifi + d
andi + 2d, ..., i + kd
(wherek
is a multiplier) are cities. If they are, unite them in UnionFind.Answer Queries: For each query
(a, b)
, simply check if they are in the same set in the UnionFind structure.
Complexity Analysis:
Time Complexity: Initializing UnionFind takes
O(n)
. Creating connections now also takesO(n log n)
due to sorting. Answering each query takes nearlyO(1)
. Forq
queries, it becomesO(q)
.Space Complexity: The UnionFind data structure takes
O(n)
space.
This optimized approach is much faster and uses less space compared to the brute force approach.
Code Explanation and Design Decisions
Identify the initial parameters and explain their significance in the context of the problem statement or the solution domain.
Discuss the primary loop or iteration over the input data. What does each iteration represent in terms of the problem you’re trying to solve? How does the iteration advance or contribute to the solution?
If there are conditions or branches within the loop, what do these conditions signify? Explain the logical reasoning behind the branching in the context of the problem’s constraints or requirements.
If there are updates or modifications to parameters within the loop, clarify why these changes are necessary. How do these modifications reflect changes in the state of the solution or the constraints of the problem?
Describe any invariant that’s maintained throughout the code, and explain how it helps meet the problem’s constraints or objectives.
Discuss the significance of the final output in relation to the problem statement or solution domain. What does it represent and how does it satisfy the problem’s requirements?
Remember, the focus here is not to explain what the code does on a syntactic level, but to communicate the intent and rationale behind the code in the context of the problem being solved.
Coding Constructs
HighLevel Strategies:
 Preprocessing with UnionFind data structure to quickly answer connectivity queries.
 Efficient population of connections using sorting and batch processing.
Purpose for NonProgrammer:
 The code is like a superefficient mailman. It first understands the quickest paths between different cities that are connected by certain rules. Later, if you ask if you can get from one city to another, it tells you instantly.
Logical Elements:
 Conditional checks (ifelse)
 Loops (for, while)
 Data structures (array, UnionFind)
Algorithmic Approach in Plain English:
 First, think of each city as its own little island.
 Then, find quick ways to connect these islands based on some math rules.
 After that, if someone asks if two cities are connected, answer them right away by just checking your list of connections.
Key Steps on Input Data:
 Initializing each city as an isolated point.
 Scanning through the cities to find connections based on divisors and then uniting connected cities.
 Responding to queries by checking the existing connections to see if two cities are connected.
Algorithmic Patterns:
 Preprocessing for quick query answering.
 UnionFind for disjointset data structure.
 Sorting and batch processing for efficient data manipulation.
 Lookup operation for quick query resolution.
Language Agnostic Coding Drills
Distinct Coding Concepts:
 Variable Initialization
 Loop Iteration
 Conditional Statements
 Array Manipulation
 Function Definition and Call
 Recursion
 Data Structures (UnionFind)
 Preprocessing (Batch Processing)
 Sorting
 Query Optimization
Coding Concepts in Order of Increasing Difficulty:
 Variable Initialization: The basis for any computation. Simple but essential.
 Loop Iteration: Requires understanding of how to repeat a block of code.
 Conditional Statements: Introduces decisionmaking but still fundamental.
 Array Manipulation: Basic data structure that requires an understanding of indexes.
 Function Definition and Call: Encapsulation of logic that can be reused.
 Recursion: A more complex form of function calls; demands a good grasp of base and recursive cases.
 Data Structures (UnionFind): A specific structure requiring understanding of disjoint sets.
 Preprocessing (Batch Processing): Requires understanding of how to prepare data for faster querying.
 Sorting: Involves complex algorithms and affects time complexity.
 Query Optimization: The pinnacle, involving multiple facets like data structures, preprocessing, and algorithmic thinking.
ProblemSolving Approach:
 Understand the Problem: Parse what the problem demands.
 Initialize Variables: Store cities, connections, and queries.
 Iterate through Data: Loop through the cities to identify possible connections.
 Use Conditionals: Apply rules to determine which cities can be connected.
 Manipulate Arrays: Store these connections in an appropriate data structure.
 Define Functions: Encapsulate logic like finding connections or answers to queries.
 Optimize with UnionFind: Use a specialized data structure to keep track of connections between cities.
 Preprocess Data: Sort and batch the data to allow faster querying.
 Implement Query Logic: Finally, implement logic to quickly answer queries based on preprocessed data.
Each coding drill trains you in a specific skill set, which cumulatively build up to the final solution. For example, understanding how to initialize variables is a prerequisite for array manipulation, which in turn is a basis for more advanced concepts like sorting and data structures.
Targeted Drills in Python
General Coding Drills
Variable Initialization
1 2
x = 5 name = "Alice"
Loop Iteration
1 2
for i in range(5): print(i)
Conditional Statements
1 2 3 4
if x > 0: print("Positive") else: print("NonPositive")
Array Manipulation
1 2
my_array = [1, 2, 3] my_array.append(4)
Function Definition and Call
1 2 3 4
def greet(name): return f"Hello, {name}" print(greet("Alice"))
Recursion
1 2 3 4
def factorial(n): if n == 1: return 1 return n * factorial(n1)
ProblemSpecific Drills
UnionFind Data Structure
1 2 3 4 5 6
parent = {} def find(x): if parent[x] != x: parent[x] = find(parent[x]) return parent[x]
Essential for keeping track of connections between cities efficiently.
Preprocessing (Batch Processing)
1 2 3
preprocessed_data = {} for item in data: preprocess(item)
Speeds up query answering by having readily available, processed data.
Sorting
1
sorted_array = sorted(my_array)
Essential for any operations requiring ordered data.
Query Optimization
1 2
def optimized_query(x): return preprocessed_data.get(x, default_value)
Makes the querying process more efficient by utilizing preprocessed data.
Integration
 Start by initializing variables that will hold your data (
Variable Initialization
).  Read the cities and connections. Use loops to go through each (
Loop Iteration
).  Apply conditional statements to validate data and connections (
Conditional Statements
).  Use arrays or other data structures to store connections and other relevant information (
Array Manipulation
).  Define functions for common operations like finding connected cities (
Function Definition and Call
).  If your logic involves recursive relationships, implement a recursive function (
Recursion
).  Use the UnionFind data structure to keep track of connected components (
UnionFind Data Structure
).  Preprocess your data to speed up queries (
Preprocessing
).  Sort data if necessary for faster access or search (
Sorting
).  Finally, build your query logic, making use of all the prior pieces (
Query Optimization
).
By following these steps in sequence, integrating each coding drill, you can build up to a complete, efficient solution for the problem.
Q&A
Similar Problems
Here are 10 LeetCode problems that share similarities with the problem we’ve discussed:
UnionFind
 684. Redundant Connection
 Uses the UnionFind data structure to identify cycles in a graph, similar to how we used it for connecting cities.
 684. Redundant Connection
Recursion
 104. Maximum Depth of Binary Tree
 Requires a recursive function to explore the tree, akin to how we might traverse connections recursively.
 104. Maximum Depth of Binary Tree
Array Manipulation
 283. Move Zeroes
 Involves manipulating an array, similar to how we manipulated arrays/lists to store city connections.
 283. Move Zeroes
Sorting
 75. Sort Colors
 Requires sorting an array for efficient manipulation, much like our problem might require sorting cities based on some metric.
 75. Sort Colors
Batch Processing/Preprocessing
 560. Subarray Sum Equals K
 Preprocessing helps to speed up the solution, akin to how we preprocessed data for quick queries.
 560. Subarray Sum Equals K
Conditionals
 13. Roman to Integer
 Uses a lot of conditional checks to convert Roman numerals, similar to how we needed to check city connections.
 13. Roman to Integer
Loop Iteration
 1. Two Sum
 Requires iterating over an array to find two numbers that add up to a target, akin to how we looped through connections.
 1. Two Sum
Function Definition and Calls
 509. Fibonacci Number
 Uses a straightforward function to calculate Fibonacci numbers, like the utility functions we defined.
 509. Fibonacci Number
Variable Initialization
 1108. Defanging an IP Address
 Requires variable initialization to store the defanged IP address, similar to our initial variables for storing city data.
 1108. Defanging an IP Address
Query Optimization
 146. LRU Cache
 Involves optimizing data retrieval, similar to how we optimized queries by preprocessing.
 146. LRU Cache
Each problem shares at least one significant concept or problemsolving strategy with our original problem, making them good for further study.