367. Valid Perfect Square

Given a positive integer num, write a function which returns True if num is a perfect square else False.

Follow up: Do not use any built-in library function such as sqrt.

class Solution {
public:
    bool isPerfectSquare(int num) {
        long long l = 1, r = INT_MAX;
        while(l <= r) {
            long long m = l + (r - l) / 2;
            long long product = m * m;
            if(product == num) return true;
            else if(product < num) l = m + 1;
            else r = m - 1;
        }
        return false;
    }
};