LeetCode:First Bad Version

問題の説明:
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.
Credits: Special thanks to @jianchao.li.fighter for adding this problem and creating all test cases.
考え方:
1、二分検索法で現在のバージョンは悪いバージョンであるが、前のバージョンは良いバージョンである.
2、境界状況の処理、特に頭尾に注意する.
3、見つからない場合は、-1を返します.
コード:

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