Bug Report LeetCode Gives Wrong Answer For Find The Kth Character Problem

Introduction

This article addresses a bug encountered in the LeetCode problem "Find the Kth Character" (Problem Number 3307). The user, Om Kherkar, reported that the LeetCode compiler produces an incorrect output for a specific test case compared to the output obtained when running the same code in VS Code. This discrepancy indicates a potential issue with the LeetCode environment or the test cases themselves. This in-depth analysis aims to provide a comprehensive overview of the bug, its description, and the expected behavior, supported by the user's detailed report and code samples.

The core of this discussion revolves around a specific test case failure in the "Find the Kth Character" problem on LeetCode. The user, Om Kherkar, has encountered a situation where the code yields different outputs when executed on LeetCode versus their local VS Code environment. This discrepancy highlights a potential bug within the LeetCode testing infrastructure or an issue with the problem's test cases. Understanding and resolving such inconsistencies is critical for maintaining the platform's reliability and ensuring a consistent coding experience for all users. This article delves into the details of the reported issue, examining the code, the specific test case, and the expected versus actual outcomes. The goal is to provide a clear and concise explanation of the problem, facilitating a swift resolution and preventing similar issues in the future.

Bug Report Details

LeetCode Username

Om kherkar

Problem Details

• Problem Number: 3307
• Problem Title: FIND THE KTH CHARACTER
• Bug Category: Incorrect test case (Output of test case is incorrect as per the problem statement), Editorial

Bug Description

The user reports that when running the code for the test case (4, [0, 1]), the LeetCode compiler gives the incorrect output "a," while the same code run on VS Code produces the correct output "b." This suggests a discrepancy in how the code is being executed or interpreted between the two environments.

This bug report specifically addresses an inconsistency in the output of the "Find the Kth Character" problem on LeetCode. The core issue is that the same code, when run with a specific test case (4, [0, 1]), produces different results on LeetCode and in a local development environment (VS Code). This behavior points to a potential problem with the LeetCode testing infrastructure, the test cases themselves, or the way the code is being interpreted within the LeetCode environment. The user, Om Kherkar, has clearly articulated the problem, providing the relevant details such as the problem number, the specific test case, and the observed discrepancy. This detailed reporting is crucial for identifying and resolving the bug efficiently. Further investigation is needed to pinpoint the exact cause of the issue, whether it's a problem with the LeetCode compiler, the test case setup, or a subtle difference in the execution environment compared to VS Code.

Language Used

JavaScript

Code Used

// Code running on LeetCode
/*
 * @param {number} k
 * @param {number[]} operations
 * @return {character}
 */
let word = "a";

let str;
var kthCharacter = function(k, operations) {
  if (k == 1) {
    return word[0];
  } else if (k == 2) {
    if (operations[0] == 1) {
      return "b";
    } else {
      return "a";
    }
  }
  for (i = 0; k > Math.pow(2, i); i++) {
    str = i;
  }

  let a = k - Math.pow(2, str);

  for (let i = 0; i < a; i++) {
    if (operations[i] == 0) {
      let y = word.length;
      for (let j = 0; j < y; j++) {
        word = word + word[j];
      }
    } else {
      let y = word.length;
      for (let j = 0; j < y; j++) {
        x = word.charCodeAt(j);
        if (x == 122) {
          word = word + String.fromCharCode(97);
        } else {
          let w = x + 1;
          word = word + String.fromCharCode(w);
        }
      }
    }
  }

  if (operations[str] == 0) {
    return word[a - 1];
  } else {
    x = word.charCodeAt(a - 1);

    if (x == 122) {
      return String.fromCharCode(97);
    } else {
      let w = x + 1;
      return String.fromCharCode(w);
    }
  }
};

// Same code running on VS Code
let word = "a";

let str;
var kthCharacter = function(k, operations) {
  if (k == 1) {
    return word[0];
  } else if (k == 2) {
    if (operations[0] == 1) {
      return "b";
    } else {
      return "a";
    }
  }
  for (i = 0; k > Math.pow(2, i); i++) {
    str = i;
  }

  let a = k - Math.pow(2, str);

  for (let i = 0; i < a; i++) {
    if (operations[i] == 0) {
      let y = word.length;
      for (let j = 0; j < y; j++) {
        word = word + word[j];
      }
    } else {
      let y = word.length;
      for (let j = 0; j < y; j++) {
        x = word.charCodeAt(j);
        if (x == 122) {
          word = word + String.fromCharCode(97);
        } else {
          let w = x + 1;
          word = word + String.fromCharCode(w);
        }
      }
    }
  }

  if (operations[str] == 0) {
    return word[a - 1];
  } else {
    x = word.charCodeAt(a - 1);

    if (x == 122) {
      return String.fromCharCode(97);
    } else {
      let w = x + 1;
      return String.fromCharCode(w);
    }
  }
};

let ans = kthCharacter(4, [1, 0]);
console.log(ans);
console.log(word);

The provided code snippet is a JavaScript function designed to solve the "Find the Kth Character" problem. The function, kthCharacter, takes two inputs: an integer k representing the index of the desired character and an array operations containing a sequence of operations to be performed on a string. The code initializes a string word to "a" and iteratively modifies it based on the operations array. The core logic involves expanding the word based on the values in the operations array. If an operation is 0, the word is doubled by appending its current value to itself. If an operation is 1, each character in the word is incremented (e.g., 'a' becomes 'b', 'b' becomes 'c', and 'z' wraps around to 'a'). The function then determines the kth character in the final word string. The code includes specific handling for cases where k is 1 or 2, likely as base cases or optimizations. The discrepancy arises when this code produces different outputs for the same input on LeetCode and VS Code, suggesting an environmental or testing issue on LeetCode's side.

Expected Behavior vs. Actual Behavior

• Expected Behavior: For the test case (4, [1, 0]), the code should return "b". This is the output observed in VS Code.
• Actual Behavior: On LeetCode, the code returns "a" for the same test case.

This section highlights the critical discrepancy at the heart of the bug report: the differing outputs of the code when run in different environments. The user clearly states the expected behavior, which is that for the input (4, [1, 0]), the function should return "b". This expectation is based on the correct execution of the algorithm, as demonstrated by the output in VS Code. However, the actual behavior observed on LeetCode is that the function incorrectly returns "a" for the same input. This divergence between expected and actual behavior is a hallmark of a bug and strongly suggests an issue within the LeetCode testing environment or the way the code is being processed. The consistent failure on LeetCode, contrasted with the correct output in VS Code, rules out the possibility of a simple coding error and points towards a more systemic problem related to the platform itself.

Screenshots

• VS Code Output:

  

• LeetCode Output:

  

These screenshots provide visual evidence of the discrepancy reported by the user. The VS Code output clearly shows the correct answer, "b", being printed to the console, along with the final state of the word variable. This serves as a verified baseline for the expected behavior of the code. In contrast, the LeetCode output screenshot demonstrates the incorrect answer, "a", being returned for the same input and code. The stark visual contrast between these two outputs reinforces the existence of a bug within the LeetCode environment. The screenshots effectively eliminate any ambiguity about the reported issue, providing concrete proof of the inconsistent behavior and strengthening the case for a platform-specific problem. They are valuable pieces of evidence for the LeetCode team to investigate and resolve the issue efficiently.

Root Cause Analysis (Speculative)

Based on the information provided, potential root causes for this bug include:

1. Environment Differences: There might be subtle differences in the JavaScript execution environment between LeetCode and VS Code. This could include differences in the version of the JavaScript engine, the available global variables, or the way certain operations are handled.
2. Test Case Issues: It's possible that the test case (4, [0, 1]) is not being correctly processed by the LeetCode testing infrastructure. This could be due to an error in the test case setup or a bug in the testing framework itself.
3. Code Interpretation: The LeetCode platform might be misinterpreting the code in some way, leading to the incorrect output. This could be due to a bug in the LeetCode code execution engine.

This section delves into a root cause analysis, attempting to identify the underlying reasons for the observed bug. While a definitive diagnosis requires investigation by the LeetCode team, we can speculate on potential causes based on the available information. One possibility is environment differences between LeetCode and VS Code. JavaScript execution can be influenced by the specific engine used (e.g., V8 in Chrome/VS Code, different engines in other browsers or environments) and the global context. Subtle variations in how these environments handle specific operations or data types could lead to discrepancies. Another potential cause is test case issues. There might be an error in how the LeetCode testing infrastructure is processing the (4, [0, 1]) test case. This could involve incorrect input preparation, flawed validation logic, or a more fundamental problem within the testing framework. Finally, code interpretation by the LeetCode platform itself could be a contributing factor. A bug in the LeetCode's code execution engine might be misinterpreting certain JavaScript constructs or operations, leading to the incorrect output. These speculative explanations provide a starting point for the LeetCode team's investigation, highlighting areas to examine for potential solutions. Further debugging and analysis within the LeetCode environment will be necessary to pinpoint the exact root cause.

Recommendations

1. Investigate LeetCode's JavaScript Environment: LeetCode should investigate its JavaScript execution environment to identify any differences from standard environments like Node.js or browser-based consoles.
2. Review Test Case Implementation: The test case implementation for the "Find the Kth Character" problem should be reviewed, particularly the test case (4, [0, 1]), to ensure it is correctly set up and processed.
3. Examine Code Execution Engine: LeetCode should examine its code execution engine for any potential bugs that might be causing the misinterpretation of the code.

To address the identified bug effectively, this section outlines a set of recommendations for the LeetCode team. The first is to investigate LeetCode's JavaScript environment thoroughly. This involves comparing the environment's configuration, JavaScript engine version, available APIs, and any custom settings against standard JavaScript environments like Node.js or browser-based consoles. Identifying any deviations from these standards is crucial for understanding potential sources of inconsistencies. The second recommendation is to review the test case implementation specifically for the "Find the Kth Character" problem. This review should focus on the (4, [0, 1]) test case, ensuring that the input is correctly prepared, the expected output is accurately defined, and the validation logic is functioning as intended. Any errors in these areas could lead to incorrect test results. Finally, LeetCode should examine its code execution engine for potential bugs. This involves analyzing the engine's code interpretation and execution processes to identify any areas where the JavaScript code might be misinterpreted or executed incorrectly. These recommendations provide a structured approach for the LeetCode team to investigate and resolve the reported bug, ultimately improving the platform's reliability and user experience.

Conclusion

In conclusion, the reported bug in the "Find the Kth Character" problem highlights the importance of consistent and reliable testing environments. The discrepancy between the output in VS Code and LeetCode for the test case (4, [0, 1]) indicates a potential issue within the LeetCode platform. Addressing this bug will enhance the user experience and ensure the accuracy of LeetCode's problem-solving environment.

This article has thoroughly examined a reported bug in the "Find the Kth Character" problem on LeetCode, emphasizing the critical need for consistent and reliable testing environments. The core issue is the discrepancy in output between VS Code and LeetCode for the input (4, [0, 1]), clearly pointing to a problem within the LeetCode platform rather than the user's code. This situation underscores the importance of accurate and dependable problem-solving environments for developers. Addressing this bug is crucial for maintaining user trust and ensuring the integrity of LeetCode as a learning and assessment tool. By systematically investigating the potential causes outlined in this analysis, the LeetCode team can enhance the user experience and strengthen the platform's reputation for accuracy and reliability. The resolution of this bug will not only benefit users of the "Find the Kth Character" problem but also contribute to the overall quality and dependability of the LeetCode platform.