LEETCODE 278

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.

利用二分查找法：

（1）如果start为错误版本，直接返回。

（2）如果mid为错误版本，那么错误版本一定在mid及mid以前，令end= mid.

（3）如果mid为正确版本，那么错误一定在mid之后，令start = mid+1.

// Forward declaration of isBadVersion API.bool isBadVersion(int version);class Solution {public:    int firstBadVersion(int n) {               int start = 1;       int end = n;       while(start < end)       {            if(isBadVersion(start) == true)                 return start;            int mid = start + (end - start)/2;            if(isBadVersion(mid) == true)                end = mid;            else                start = mid + 1;       }       return start;    }};