leetcode278第一个错误的版本(FirstBadVersion)

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

class Solution {
public:
    int firstBadVersion(int n) {
        if(isBadVersion(1)){
            return 1;
        }
        int bg=1;
        int mid= (1+n)/2;
        while(n-bg>1){
            if(isBadVersion(mid)){
                n=mid;
            }
            else{
                bg=mid;
            }
            mid= bg+(n-bg)/2;
        }
        return n;
    }
};