First Bad Version

原题：
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.

解题：
解题的话比较简单，直接用二分就可以，但是需要找到第一次出现bad的元素的下标。在求取mid的时候需要用mid = left + (right - left) / 2,而不是mid=(left + right) / 2这种形式。
可以AC的C++代码如下：

bool isBadVersion(int version);

int firstBadVersion(int n) {
    int left = 1, right = n;
    int mid = 0;

    while(left < right){
        mid = left + (right - left)/2;
        if(isBadVersion(mid)) {
            right = mid;
        }else{
            left = mid + 1;
        }
    }

    if(isBadVersion(left)) 
        return left;

    return 0;
}