解題思路: 牛頓迭代法, https://en.wikipedia.org/wiki/Integer_square_root#Using_only_integer_division

點擊打開鏈接

class Solution {public:    int mySqrt(int x) {        long long r = x;        while (r*r > x)            r = (r + x/r) / 2;        return r;    }};