278. First Bad Version

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.

s思路:
1. 你看这道题说得还是一件事，分组。看下图，有个边界，左边都是good，右边都是bad。就是分两组，每组都是一样，这不就是已经排好序了吗？所以直接上binary search！

2. 突然想到binary search怎么个灵活的？为了handle一个vector,由于vector是两个端点，也就是两个边界，因此左右各放一个哨兵，就可以看到如何变化的，而且这两个哨兵还是独立的运动。有趣！

//方法：binary search// Forward declaration of isBadVersion API.bool isBadVersion(int version);class Solution {public:    int firstBadVersion(int n) {        //        int l=1,r=n;        while(l<=r){            int m=l+(r-l)/2;            bool cur=isBadVersion(m);            if(cur) r=m-1;            else l=m+1;            }        return l;    }};