class Solution {
public:
    int mySqrt(int x) {
        int l = 0, r = x;
        while (l < r) {
            long long mid = (long long )l + r + 1 >> 1; //r可能越界 int mid = l + 1ll + r >> 1;
            if (mid == x / mid) return mid;
            else if (mid < x / mid) l = mid;
            else r = mid - 1; 
        }
        return l;
    }
};