278.First Bad Version

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.

版本序列已经sorted，又要求尽可能少调用API，所以最好的的办法就是Binary-Search，不过和常规的B-search相比，这里要做一些变化，当检测到mid版本有问题时，则说明应该去左半区继续查找;否则应该去右半区。最后返回low指针指向的版本号即可，代码如下:

// Forward declaration of isBadVersion API.
bool isBadVersion(int version);

class Solution {
public:
    int firstBadVersion(int n) {
        int low=0,high=n-1,mid=0;
        while(low<=high){
            mid=low+(high-low)/2;
            if(isBadVersion(mid))
                high=mid-1;
            else
                low=mid+1;
        }
        return low;
    }
};