[leetcode] 368. Largest Divisible Subset

class Solution {public:    bool isPerfectSquare(int num) {        long long low = 0,            high = num;        while ( low <= high){            long long mid= (low + high) / 2;            if ( mid * mid == num)                return true;            else if (mid * mid < num)                low = mid + 1;            else high = mid -1;        }        return false;    }};