278. First Bad Version

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

class Solution {
public:
    int firstBadVersion(int n) 
    {
        int s = 1, e = n;
        int mid =  s + (e-s)/2;

        while(s<=e)
        {
            
            int ans = isBadVersion(mid);

            if(ans == 0)
                s = mid + 1;
            else
                e = mid - 1;
            mid = s + (e-s)/2;
        }

        return mid;
    }
};