278.first-bad-version.cpp

/*
 * @lc app=leetcode id=278 lang=cpp
 *
 * [278] First Bad Version
 *
 * https://leetcode.com/problems/first-bad-version/description/
 *
 * algorithms
 * Easy (29.01%)
 * Total Accepted:    205.2K
 * Total Submissions: 706.8K
 * Testcase Example:  '5\n4'
 *
 * 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 will return whether
 * version is bad. Implement a function to find the first bad version. You
 * should minimize the number of calls to the API.
 *
 * Example:
 *
 *
 * Given n = 5, and version = 4 is the first bad version.
 *
 * call isBadVersion(3) -> false
 * call isBadVersion(5) -> true
 * call isBadVersion(4) -> true
 *
 * Then 4 is the first bad version. 
 *
 */
// Forward declaration of isBadVersion API.
bool isBadVersion(int version);

class Solution
{
  public:
    int firstBadVersion(int n)
    {
        if (n == 0)
            return -1;

        int left = 1, right = n;
        while (left < right)
        {
            // Prevent (left + right) overflow
            int mid = left + (right - left) / 2;
            if (isBadVersion(mid))
                right = mid;
            else
                left = mid + 1;
        }

        // Post-processing:
        // End Condition: left == right
        if (isBadVersion(left))
            return left;
        return -1;
    }
};