First Bad Version

<-E 278> First Bad Version

// The API isBadVersion is defined for you.
// bool isBadVersion(int version);

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