Count Number of Texts
We use tabulation dp to store the number of possible messages for i characters.
For i + 1th character, the number of messages is dp[i + 1] = dp[i], plus:
dp[i  1] if key[i + 1] == key[i]
dp[i  2] if key[i + 1] == key[i  1]
dp[i  3] if key[i + 1] == key[i  2]
dp[i  4] if key[i + 1] == key[i  3] and key[i + 1] is either 7 or 9.
We only need to store the tabulation for 4 previous positions. We can use a circular array to achieve O(1) memory complexity.


Identifying Problem Isomorphism
“Count Number of Texts” can be approximately mapped to “Decode Ways”.
In both problems, the idea is interpreting a string of numbers, although the interpretation and constraints are different.
“Decode Ways” is about interpreting a string of numbers into possible alphabets (with a mapping from 126 to AZ), while “Count Number of Texts” is about interpreting a string of pressed keys into possible text messages (with each number from 29 mapping to a set of letters based on a phone keypad).
“Decode Ways” is simpler in terms of the constraints, as there’s only one possible character for each number, whereas “Count Number of Texts” deals with multiple characters per number and needs to handle the modulo operation due to larger numbers.
While they share similar concepts of decoding and counting possibilities, the implementations will vary significantly due to the different specifics and constraints in each problem.
10 Prerequisite LeetCode Problems
For this, the following are a good preparation:
“20. Valid Parentheses”  This problem helps in understanding how a stack can help in processing sequences in order, similar to the processing of keys in the main problem.
“344. Reverse String”  This problem gives a good grasp of how to manipulate strings, which is crucial for the main problem.
“387. First Unique Character in a String”  This problem strengthens skills in string processing and counting occurrences, similar to the main problem.
“125. Valid Palindrome”  This problem also deals with string manipulations and can help build a better understanding of string processing.
“409. Longest Palindrome”  It helps you understand how frequency counts can be used to build desired outcomes.
“647. Palindromic Substrings”  This problem can help you think about counting possible outcomes based on sequences.
“3. Longest Substring Without Repeating Characters”  This problem helps in understanding sequences and how to count unique combinations.
“139. Word Break”  This problem is about forming words from given sequences, similar to the main problem.
“17. Letter Combinations of a Phone Number”  This problem is about enumerating possibilities from keypresses on a telephone, directly related to the main problem.
“91. Decode Ways”  This problem involves interpreting a sequence in multiple ways, similar to the main problem.
These cover string manipulations, sequence processing, and enumeration of possibilities, which are useful for the main problem.
Clarification Questions
What are the clarification questions we can ask about this problem?
Problem Analysis and Key Insights
What are the key insights from analyzing the problem statement?
Problem Boundary
What is the scope of this problem?
How to establish the boundary of this problem?
Problem Classification
Problem Statement:Alice is texting Bob using her phone. The mapping of digits to letters is shown in the figure below.
In order to add a letter, Alice has to press the key of the corresponding digit i times, where i is the position of the letter in the key.
For example, to add the letter ’s’, Alice has to press ‘7’ four times. Similarly, to add the letter ‘k’, Alice has to press ‘5’ twice. Note that the digits ‘0’ and ‘1’ do not map to any letters, so Alice does not use them. However, due to an error in transmission, Bob did not receive Alice’s text message but received a string of pressed keys instead.
For example, when Alice sent the message “bob”, Bob received the string “2266622”. Given a string pressedKeys representing the string received by Bob, return the total number of possible text messages Alice could have sent.
Since the answer may be very large, return it modulo 109 + 7.
Example 1:
Input: pressedKeys = “22233” Output: 8 Explanation: The possible text messages Alice could have sent are: “aaadd”, “abdd”, “badd”, “cdd”, “aaae”, “abe”, “bae”, and “ce”. Since there are 8 possible messages, we return 8. Example 2:
Input: pressedKeys = “222222222222222222222222222222222222” Output: 82876089 Explanation: There are 2082876103 possible text messages Alice could have sent. Since we need to return the answer modulo 109 + 7, we return 2082876103 % (109 + 7) = 82876089.
Constraints:
1 <= pressedKeys.length <= 105 pressedKeys only consists of digits from ‘2’  ‘9’.
Analyze the provided problem statement. Categorize it based on its domain, ignoring ‘How’ it might be solved. Identify and list out the ‘What’ components. Based on these, further classify the problem. Explain your categorizations.
Distilling the Problem to Its Core Elements
Can you identify the fundamental concept or principle this problem is based upon? Please explain. What is the simplest way you would describe this problem to someone unfamiliar with the subject? What is the core problem we are trying to solve? Can we simplify the problem statement? Can you break down the problem into its key components? What is the minimal set of operations we need to perform to solve this problem?
Visual Model of the Problem
How to visualize the problem statement for this problem?
Problem Restatement
Could you start by paraphrasing the problem statement in your own words? Try to distill the problem into its essential elements and make sure to clarify the requirements and constraints. This exercise should aid in understanding the problem better and aligning our thought process before jumping into solving it.
Abstract Representation of the Problem
Could you help me formulate an abstract representation of this problem?
Given this problem, how can we describe it in an abstract way that emphasizes the structure and key elements, without the specific realworld details?
Terminology
Are there any specialized terms, jargon, or technical concepts that are crucial to understanding this problem or solution? Could you define them and explain their role within the context of this problem?
Problem Simplification and Explanation
Could you please break down this problem into simpler terms? What are the key concepts involved and how do they interact? Can you also provide a metaphor or analogy to help me understand the problem better?
Constraints
Given the problem statement and the constraints provided, identify specific characteristics or conditions that can be exploited to our advantage in finding an efficient solution. Look for patterns or specific numerical ranges that could be useful in manipulating or interpreting the data.
What are the key insights from analyzing the constraints?
Case Analysis
Could you please provide additional examples or test cases that cover a wider range of the input space, including edge and boundary conditions? In doing so, could you also analyze each example to highlight different aspects of the problem, key constraints and potential pitfalls, as well as the reasoning behind the expected output for each case? This should help in generating key insights about the problem and ensuring the solution is robust and handles all possible scenarios.
Provide names by categorizing these cases
What are the edge cases?
How to visualize these cases?
What are the key insights from analyzing the different cases?
Identification of Applicable Theoretical Concepts
Can you identify any mathematical or algorithmic concepts or properties that can be applied to simplify the problem or make it more manageable? Think about the nature of the operations or manipulations required by the problem statement. Are there existing theories, metrics, or methodologies in mathematics, computer science, or related fields that can be applied to calculate, measure, or perform these operations more effectively or efficiently?
Simple Explanation
Can you explain this problem in simple terms or like you would explain to a nontechnical person? Imagine you’re explaining this problem to someone without a background in programming. How would you describe it? If you had to explain this problem to a child or someone who doesn’t know anything about coding, how would you do it? In layman’s terms, how would you explain the concept of this problem? Could you provide a metaphor or everyday example to explain the idea of this problem?
Problem Breakdown and Solution Methodology
Given the problem statement, can you explain in detail how you would approach solving it? Please break down the process into smaller steps, illustrating how each step contributes to the overall solution. If applicable, consider using metaphors, analogies, or visual representations to make your explanation more intuitive. After explaining the process, can you also discuss how specific operations or changes in the problem’s parameters would affect the solution? Lastly, demonstrate the workings of your approach using one or more example cases.
Inference of ProblemSolving Approach from the Problem Statement
Can you identify the key terms or concepts in this problem and explain how they inform your approach to solving it? Please list each keyword and how it guides you towards using a specific strategy or method. How can I recognize these properties by drawing tables or diagrams?
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
Could you please provide a stepwise refinement of our approach to solving this problem?
How can we take the highlevel solution approach and distill it into more granular, actionable steps?
Could you identify any parts of the problem that can be solved independently?
Are there any repeatable patterns within our solution?
Solution Approach and Analysis
Given the problem statement, can you explain in detail how you would approach solving it? Please break down the process into smaller steps, illustrating how each step contributes to the overall solution. If applicable, consider using metaphors, analogies, or visual representations to make your explanation more intuitive. After explaining the process, can you also discuss how specific operations or changes in the problem’s parameters would affect the solution? Lastly, demonstrate the workings of your approach using one or more example cases.
Identify Invariant
What is the invariant in this problem?
Identify Loop Invariant
What is the loop invariant in this problem?
Is invariant and loop invariant the same for this problem?
Thought Process
Can you explain the basic thought process and steps involved in solving this type of problem?
Explain the thought process by thinking step by step to solve this problem from the problem statement and code the final solution. Write code in Python3. What are the cues in the problem statement? What direction does it suggest in the approach to the problem? Generate insights about the problem statement.
Establishing Preconditions and Postconditions
Parameters:
 What are the inputs to the method?
 What types are these parameters?
 What do these parameters represent in the context of the problem?
Preconditions:
 Before this method is called, what must be true about the state of the program or the values of the parameters?
 Are there any constraints on the input parameters?
 Is there a specific state that the program or some part of it must be in?
Method Functionality:
 What is this method expected to do?
 How does it interact with the inputs and the current state of the program?
Postconditions:
 After the method has been called and has returned, what is now true about the state of the program or the values of the parameters?
 What does the return value represent or indicate?
 What side effects, if any, does the method have?
Error Handling:
 How does the method respond if the preconditions are not met?
 Does it throw an exception, return a special value, or do something else?
Problem Decomposition
Problem Understanding:
 Can you explain the problem in your own words? What are the key components and requirements?
Initial Breakdown:
 Start by identifying the major parts or stages of the problem. How can you break the problem into several broad subproblems?
Subproblem Refinement:
 For each subproblem identified, ask yourself if it can be further broken down. What are the smaller tasks that need to be done to solve each subproblem?
Task Identification:
 Within these smaller tasks, are there any that are repeated or very similar? Could these be generalized into a single, reusable task?
Task Abstraction:
 For each task you’ve identified, is it abstracted enough to be clear and reusable, but still makes sense in the context of the problem?
Method Naming:
 Can you give each task a simple, descriptive name that makes its purpose clear?
Subproblem Interactions:
 How do these subproblems or tasks interact with each other? In what order do they need to be performed? Are there any dependencies?
From Brute Force to Optimal Solution
Could you please begin by illustrating a brute force solution for this problem? After detailing and discussing the inefficiencies of the brute force approach, could you then guide us through the process of optimizing this solution? Please explain each step towards optimization, discussing the reasoning behind each decision made, and how it improves upon the previous solution. Also, could you show how these optimizations impact the time and space complexity of our solution?
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
Consider the code for the solution of this problem.
What are the highlevel problemsolving strategies or techniques being used by this code?
If you had to explain the purpose of this code to a nonprogrammer, what would you say?
Can you identify the logical elements or constructs used in this code, independent of any programming language?
Could you describe the algorithmic approach used by this code in plain English?
What are the key steps or operations this code is performing on the input data, and why?
Can you identify the algorithmic patterns or strategies used by this code, irrespective of the specific programming language syntax?
Language Agnostic Coding Drills
Your mission is to deconstruct this code into the smallest possible learning units, each corresponding to a separate coding concept. Consider these concepts as unique coding drills that can be individually implemented and later assembled into the final solution.
Dissect the code and identify each distinct concept it contains. Remember, this process should be languageagnostic and generally applicable to most modern programming languages.
Once you’ve identified these coding concepts or drills, list them out in order of increasing difficulty. Provide a brief description of each concept and why it is classified at its particular difficulty level.
Next, describe the problemsolving approach that would lead from the problem statement to the final solution. Think about how each of these coding drills contributes to the overall solution. Elucidate the stepbystep process involved in using these drills to solve the problem. Please refrain from writing any actual code; we’re focusing on understanding the process and strategy.
Targeted Drills in Python
Now that you’ve identified and ordered the coding concepts from a complex software code in the previous exercise, let’s focus on creating Pythonbased coding drills for each of those concepts.
Begin by writing a separate piece of Python code that encapsulates each identified concept. These individual drills should illustrate how to implement each concept in Python. Please ensure that these are suitable even for those with a basic understanding of Python.
In addition to the general concepts, identify and write coding drills for any problemspecific concepts that might be needed to create a solution. Describe why these drills are essential for our problem.
Once all drills have been coded, describe how these pieces can be integrated together in the right order to solve the initial problem. Each drill should contribute to building up to the final solution.
Remember, the goal is to not only to write these drills but also to ensure that they can be cohesively assembled into one comprehensive solution.
Q&A


The specific rules are encoded in the logic of the code. Here’s an explanation of the code:
Initialization:
 An array
dp
of 5 integers is initialized with all elements as 1. This array os used for dynamic programming to store some intermediate results.  Variable
n
is assigned the length of the stringk
.
 An array
Main Loop:
 A loop is run from ( n  1 ) down to 0 (inclusive). This loop iterates over each character in the string
k
from right to left.  Inside the loop, the value at
dp[i % 5]
is reset to 0. The modulo operation is used to cycle through the 5 values indp
, creating a rolling window.  The variable
max_j
is calculated based on the current character atk[i]
. If the character is either ‘7’ or ‘9’,max_j
will be one more than otherwise. This might be related to a specific rule of the problem.  An inner loop runs from
i
tomax_j  1
(inclusive) and incrementsdp[i % 5]
bydp[(j + 1) % 5]
ifk[i]
is equal tok[j]
. The result is taken modulo ( 1000000007 ), a common practice to prevent integer overflow.
 A loop is run from ( n  1 ) down to 0 (inclusive). This loop iterates over each character in the string
Return Statement:
 The function returns
dp[0]
, which is the final result of the computation.
 The function returns
Key Takeaways
 The code uses dynamic programming with a rolling window of size 5.
 It processes the input string from right to left.
 Specific handling for characters ‘7’ and ‘9’ is included, which relate to the rules of the problem.
Similar Problems
Can you suggest 10 problems from LeetCode that require similar problemsolving strategies or use similar underlying concepts as the problem we’ve just solved? These problems can be from any domain or topic, but they should involve similar steps or techniques in the solution process. Also, please briefly explain why you consider each of these problems to be related to our original problem. The response text is of the following format:
Here are 10 problems that use similar underlying concepts: