/* The isBadVersion API is defined in the parent class VersionControl.      bool IsBadVersion(int version); */public class Solution : VersionControl {    public int FirstBadVersion(int n) {        var start = 1;            var end = n;            while (start < end)            {                var mid = start + (end - start) / 2;                if (!IsBadVersion(mid))                {                    start = mid + 1;                }                else                {                    end = mid;                }                            }            return start;    }}