class Solution {    public int mySqrt(int x) {        if (x < 0) {            return -1;        }                if (x < 2) {            return x;        }        int start = 1;        int end = x;        while (end - start > 1) {            int mid = start + (end - start)/2;            if (mid == x / mid) {                return mid;            } else if (mid > x / mid) {                end = mid;            } else {                start = mid;            }        }        return start;    }}