LeetCode Bug: Missing Test Case In String Search Problem

Hey guys! Let's dive into a fascinating discussion about a missing test case we've spotted in the LeetCode problem, "Find the Index of the First Occurrence in a String." This is super crucial because it affects how we evaluate the correctness and efficiency of our code. Let's break it down!

LeetCode Feedback: The Heart of the Issue

LeetCode is an awesome platform for honing our coding skills, but sometimes, even the best systems can have blind spots. This particular issue falls under the LeetCode-Feedback category, highlighting a missing test case that can lead to incorrect or inefficient code slipping through the cracks. It’s all about making the platform even better by identifying and addressing these gaps. Let's collaborate and make LeetCode even more robust!

Problem Details: Zeroing In

We're focusing on Problem Number 28, famously known as "Find the Index of the First Occurrence in a String." You can find it right here: https://leetcode.com/problems/find-the-index-of-the-first-occurrence-in-a-string. The core challenge is to locate the starting index of a given needle string within a larger haystack string. A classic string searching puzzle, right?

Bug Alert: Missing the Mark

The Bug Category here is a Missing test case. This means that the existing test suite doesn't fully cover all possible scenarios, allowing potentially flawed solutions to be accepted. Think of it like this: our code might pass all the visible tests, but a hidden edge case could trip it up. Identifying these gaps is key to writing robust and reliable code.

Bug Description: Spotting the Flaw

The bug surfaces when the haystack is "hellowoworldthere" and the needle is "world". Sounds simple, but it exposes a weakness in some common approaches. Many solutions might fail to correctly identify the starting index of "world" within "hellowoworldthere".

Code in the Spotlight: C++ Implementation

The language used in this scenario is C++, a powerful and versatile choice for algorithm challenges. C++ lets us get down to the metal, optimizing for performance while maintaining readability. Let's look at the code snippet that brings this issue to light.

int strStr(string haystack, string needle) {
    int k=0;
    for(int i=0;i<haystack.size();i++){
        if(haystack[i]==needle[k]){
            k++;
            if(k==needle.size()) return i-k+1;
        }
        else {
            i-=k;
            k=0;
        }
    }
    return -1;
}

This code iterates through the haystack, comparing characters with the needle. It uses a clever approach to reset the search if a mismatch is found. However, it’s this very logic that can falter in certain situations, like the one we’ve highlighted.

Expected Behavior: The Correct Answer

In this specific case, the Expected behavior is that the code should return 7. This is because the substring "world" starts at index 7 within the string "hellowoworldthere". Getting this right is crucial for a correct and robust solution.

Deep Dive into the Issue: Analyzing the Code and the Missing Test Case

To truly understand this bug, let's dissect the provided C++ code and see why it fails for the given input. The core of the function strStr lies in its iterative comparison of characters between the haystack and the needle. The variable k acts as a pointer, tracking the current position within the needle string.

1. Initial Scan: The loop starts, and the code checks each character of the haystack against the first character of the needle. When a match is found (haystack[i] == needle[k]), k increments, moving to the next character in the needle.
2. Complete Match: If k reaches the length of the needle, it means a complete match has been found. The function then returns the starting index (i - k + 1).
3. Mismatch Handling: The tricky part is the else block. If a mismatch occurs, the code attempts to reset the search. It decrements i by k and resets k to 0. This is where the logic can stumble.

In our case, with `haystack =