Count Pairs With XOR in a Range

The given problem is about finding the number of nice pairs in the integer array nums where a nice pair is defined as a pair (i, j) satisfying ( 0 \leq i < j < \text{nums.length} ) and ( \text{low} \leq (\text{nums}[i] \text{ XOR} \text{nums}[j]) \leq \text{high} ).

Step 1: Understand the Problem

We are given an integer array, and we need to find the number of pairs whose XOR result is within the given bounds low and high.

Step 2: Approach

To efficiently count the number of nice pairs, we can use a Trie data structure. This will help us quickly find the count of numbers in the array that will have an XOR result within the given bounds for a particular number.

Step 3: Trie Implementation

We’ll build a Trie using the binary representations of the numbers in the array, and we will store the count of numbers having the same bits at each node.

Step 4: Counting Pairs

For each number in the array, we will traverse the Trie to count the number of numbers whose XOR with the current number will be within the bounds low and high.

Code

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class TrieNode:
    def __init__(self):
        self.children = {}
        self.count = 0

class Solution:
    def countPairs(self, nums: List[int], low: int, high: int) -> int:
        def add(number):
            node = root
            for i in range(15, -1, -1):
                bit = (number >> i) & 1
                if bit not in node.children:
                    node.children[bit] = TrieNode()
                node = node.children[bit]
                node.count += 1

        def countPairsWithXORLessThan(number, bound):
            node = root
            count = 0
            for i in range(15, -1, -1):
                bit = (number >> i) & 1
                boundBit = (bound >> i) & 1
                if boundBit:
                    if bit in node.children:
                        count += node.children[bit].count
                    if 1 - bit in node.children:
                        node = node.children[1 - bit]
                    else:
                        break
                else:
                    if bit in node.children:
                        node = node.children[bit]
                    else:
                        break
            return count

        root = TrieNode()
        for num in nums:
            add(num)

        pairs = 0
        for num in nums:
            pairs += countPairsWithXORLessThan(num, high + 1)
            pairs -= countPairsWithXORLessThan(num, low)
        
        return pairs // 2

Key Takeaways

  • We used a Trie to efficiently represent the numbers in binary form.
  • We constructed the Trie and counted the pairs satisfying the given conditions using two separate functions.
  • The main function iterated through the numbers, and for each number, it counted the pairs whose XOR is less than high + 1 and subtracted the count of pairs whose XOR is less than low. Finally, we divided the result by 2 to account for the double counting of pairs.

Identifying Problem Isomorphism

“Count Pairs With XOR in a Range” can be mapped to “Subarray XOR Less Than K”.

Reasoning:

Both problems involve exploring pairs within an array and evaluating their XOR. “Count Pairs With XOR in a Range” asks us to find how many pairs have an XOR value within a given range, while “Subarray XOR Less Than K” is about the number of subarrays having XOR less than K.

The difference is that “Count Pairs With XOR in a Range” looks for pairs, while “Subarray XOR Less Than K” counts subarrays of varying length. Both problems share a common theme of counting specific configurations within an array based on XOR operations, which can be solved via the use of a Trie data structure optimized for binary representations.

“Count Pairs With XOR in a Range” is more challenging because it asks for XOR within a specific range, not just less than a value. Hence, it requires a more complex implementation than “Subarray XOR Less Than K”.

10 Prerequisite LeetCode Problems

“Count Pairs With XOR in a Range” involves bitwise operations and a trie data structure. Here are 10 problems to prepare for tackling this problem:

  1. “136. Single Number”

    • This problem introduces the basic concept of XOR operation.

  2. “191. Number of 1 Bits”

    • This problem involves bit manipulation, which is fundamental to understand bitwise XOR operation.

  3. “231. Power of Two”

    • This problem reinforces the concept of bitwise operation.

  4. “260. Single Number III”

    • Another problem on the application of XOR operation.

  5. “421. Maximum XOR of Two Numbers in an Array”

    • This problem also requires knowledge of XOR operation and introduces Trie for bitwise operation.

  6. “137. Single Number II”

    • This problem is another variation of Single Number problem which involves bitwise operation.

  7. “338. Counting Bits”

    • This problem requires bit manipulation and dynamic programming to solve.

  8. “190. Reverse Bits”

    • This problem is to reverse the bits in a given 32-bit unsigned integer.

  9. “201. Bitwise AND of Numbers Range”

    • This problem involves the bitwise AND operation which can be helpful to understand bitwise operations in general.

  10. “187. Repeated DNA Sequences”

    • This problem introduces the concept of a trie, which is useful for “Count Pairs With XOR in a Range”.

By understanding and solving these problems, you will have a better grasp of the concepts that are crucial for solving the “Count Pairs With XOR in a Range” problem.

Problem Classification

Problem Statement: 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.

Clarification Questions

What are the clarification questions we can ask about this problem?

Identifying Problem Isomorphism

Distilling the Problem to Its Core Elements

In order to have me distill a problem to its core, you could ask questions that prompt for a deeper analysis of the problem, understanding of the underlying concepts, and simplification of the problem’s essence. Here are some examples of such prompts:

  1. Can you identify the fundamental concept or principle this problem is based upon? Please explain.
  2. What is the simplest way you would describe this problem to someone unfamiliar with the subject?
  3. What is the core problem we are trying to solve? Can we simplify the problem statement?
  4. Can you break down the problem into its key components?
  5. What is the minimal set of operations we need to perform to solve this problem?

These prompts guide the discussion towards simplifying the problem, stripping it down to its essential elements, and understanding the core problem to be solved.

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?

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 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?

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?

Code Explanation and Design Decisions

  1. Identify the initial parameters and explain their significance in the context of the problem statement or the solution domain.

  2. 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?

  3. 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.

  4. 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?

  5. Describe any invariant that’s maintained throughout the code, and explain how it helps meet the problem’s constraints or objectives.

  6. 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 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.

Q&A

Similar Problems

Can you suggest 10 problems from LeetCode that require similar problem-solving 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.