Leetcode题目：First Bad Version

题目：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.

题目解答：这个题需要最小化对API的调用，其实就是减少判断的数目。考虑使用二分的思路。另外，在求二分的中位时，需要注意不要越界，使用long long表示。

代码如下：

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

class Solution {
public:
    int firstBadVersion(int n) {
        long long i = 1;
        long long j = n;
        long long mid_num = n / 2;
        while((i < j - 1)  )
        {
            if(isBadVersion(mid_num) )
            {
                j = mid_num;
                mid_num = (i + j) / 2;
            }
            else
            {
                i = mid_num;
                mid_num = (i + j) / 2;
            }
        }
        if(isBadVersion(i) )
        {
            return i;
        }
        else
            return j;
       
    }
};