278. First Bad Version

At a Glance

• Topic: Binary Search
• Pattern: Binary Search
• Difficulty: Easy
• LeetCode: 278

Problem Statement

You are a product manager and currently leading a team to develop a new product. Unfortunately, the latest version of your product fails the quality check. Since each version is developed based on the previous version, all the versions after a bad version are also bad.

Suppose you have n versions [1, 2, ..., n] and you want to find out the first bad one, which causes all the following ones to be bad.

You are given an API bool isBadVersion(version) which returns whether version is bad. Implement a function to find the first bad version. You should minimize the number of calls to the API.

Example 1:

Input: n = 5, bad = 4 Output: 4 Explanation: call isBadVersion(3) -> false call isBadVersion(5) -> true call isBadVersion(4) -> true Then 4 is the first bad version.

Example 2:

Input: n = 1, bad = 1 Output: 1

Constraints:

1 <= bad <= n <= 231 - 1

Approach & Solution Steps

Use binary search to find the transition point from good to bad. If the mid version is bad, search the left half. If it's good, search the right half.

Optimal JS Solution

var solution = function(isBadVersion) {
  return function(n) {
    let left = 1, right = n;
    while (left < right) {
      const mid = left + Math.floor((right - left) / 2);
      if (isBadVersion(mid)) right = mid;
      else left = mid + 1;
    }
    return left;
  };
};

Edge Cases & Pitfalls

• Always consider empty or null inputs.
• Watch out for off-by-one index errors.