Three Equal Parts

class Solution:
    def threeEqualParts(self, A):
        n = len(A)
        indexes = [i for i in range(n) if A[i] == 1]
        m = len(indexes)

        if m == 0: return [0, 2]

        if m % 3 != 0: return [-1, -1]
        p1, p2 = indexes[0], indexes[m//3-1]
        p3, p4 = indexes[m//3], indexes[2*m//3-1]
        p5, p6 = indexes[2*m//3], indexes[-1]
        part1, part2, part3 = A[p1:p2+1], A[p3:p4+1], A[p5:p6+1]

        if part1 != part2 or part2 != part3: return [-1, -1]

        l1 = p3 - p2 - 1
        l2 = p5 - p4 - 1
        l3 = n - p6 - 1

        if l3 > l2 or l3 > l1: return [-1, -1]

        return [p2 + l3, p4 + l3 + 1]

Identifying Problem Isomorphism

“Three Equal Parts” has an approximate isomorphism “Split Array into Consecutive Subsequences”. Both problems require you to divide an array into specific parts under given conditions.

In “Three Equal Parts”, you have to split a binary array into three non-empty parts that represent the same binary value. In other words, the binary values resulting from the three partitions should be the same.

In “Split Array into Consecutive Subsequences”, you have to divide an array of integers into one or more subsequences of consecutive numbers. The goal is to determine whether it is possible to do so such that each subsequence has a length of at least 3.

Although the conditions for splitting the array are different in each case, both problems share the same fundamental requirement of partitioning an array into separate parts that meet certain conditions.

“Three Equal Parts” is more challenging due to the need to handle binary values and ensure all three parts are equivalent. Both problems require the use of array manipulation and algorithmic problem-solving skills.

10 Prerequisite LeetCode Problems

“927. Three Equal Parts” requires understanding of bit manipulation and array manipulation techniques. Here are some simpler problems to prepare for this:

  1. 136. Single Number: An introduction to using XOR for bit manipulation.

  2. 191. Number of 1 Bits: It helps you understand how to count bits in a binary representation.

  3. 338. Counting Bits: This problem involves dynamic programming with bit manipulation.

  4. 78. Subsets: It helps you understand how to generate all subsets, a critical step in solving the three equal parts problem.

  5. 231. Power of Two: This problem reinforces understanding of bit manipulation and binary representation.

  6. 268. Missing Number: It uses bit manipulation to find the missing number.

  7. 201. Bitwise AND of Numbers Range: This problem enhances understanding of bit manipulation over a range of numbers.

  8. 190. Reverse Bits: It helps understand bit reversal, a step that might be useful in array manipulation.

  9. 260. Single Number III: This problem requires more complex bit manipulation compared to the previous single number problem.

  10. 289. Game of Life: This problem introduces in-place array manipulation techniques which can be useful for the problem at hand.

Problem Classification

Problem Statement: You are given an array arr which consists of only zeros and ones, divide the array into three non-empty parts such that all of these parts represent the same binary value. If it is possible, return any [i, j] with i + 1 < j, such that: arr[0], arr[1], …, arr[i] is the first part, arr[i + 1], arr[i + 2], …, arr[j - 1] is the second part, and arr[j], arr[j + 1], …, arr[arr.length - 1] is the third part. All three parts have equal binary values. If it is not possible, return [-1, -1]. Note that the entire part is used when considering what binary value it represents. For example, [1,1,0] represents 6 in decimal, not 3. Also, leading zeros are allowed, so [0,1,1] and [1,1] represent the same value.

Example 1:

Input: arr = [1,0,1,0,1] Output: [0,3] Example 2:

Input: arr = [1,1,0,1,1] Output: [-1,-1] Example 3:

Input: arr = [1,1,0,0,1] Output: [0,2]


3 <= arr.length <= 3 * 104 arr[i] is 0 or 1

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.

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 real-world details?


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?


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.

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?

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 Problem-Solving Approach from the Problem Statement

How did you infer from the problem statement that this problem can be solved using ?

Stepwise Refinement

  1. Could you please provide a stepwise refinement of our approach to solving this problem?

  2. How can we take the high-level solution approach and distill it into more granular, actionable steps?

  3. Could you identify any parts of the problem that can be solved independently?

  4. 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.

Thought Process

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.

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?

Coding Constructs

Consider the following piece of complex software code.

  1. What are the high-level problem-solving strategies or techniques being used by this code?

  2. If you had to explain the purpose of this code to a non-programmer, what would you say?

  3. Can you identify the logical elements or constructs used in this code, independent of any programming language?

  4. Could you describe the algorithmic approach used by this code in plain English?

  5. What are the key steps or operations this code is performing on the input data, and why?

  6. 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.

  1. Dissect the code and identify each distinct concept it contains. Remember, this process should be language-agnostic and generally applicable to most modern programming languages.

  2. 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.

  3. Next, describe the problem-solving 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 step-by-step 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 Python-based coding drills for each of those concepts.

  1. 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.

  2. In addition to the general concepts, identify and write coding drills for any problem-specific concepts that might be needed to create a solution. Describe why these drills are essential for our problem.

  3. 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.


Similar Problems

Given the problem , identify and list down 10 similar problems on LeetCode. These should cover similar concepts or require similar problem-solving approaches as the provided problem. Please also give a brief reason as to why you think each problem is similar to the given problem.