【leetcode】367. Valid Perfect Square

題目：
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.

---

思路：
我是用的最笨的方法，實際上其背後蘊含着一個數學規律。

---

代碼實現：

class Solution {
public:
    bool isPerfectSquare(int num) {
        long i = 1;
        while (i*i < num){
            ++i;
        }
        
        return i*i == num;
    }
};