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

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

class Solution {
public:
    bool isPerfectSquare(int num) {
        if (num < 0) return false;
        if (num == 0 || num == 1) return true;
        for (int i = 1; i <= num / i; i++) {
            if (i * i == num) return true;
        }
        return false;
    }
};