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.

Example 1:

Input: 16
Output: true

Example 2:

Input: 14
Output: false

想法：完全平方數是等差數列相加。

class Solution {
public:
    bool isPerfectSquare(int
 
 num) {
        int x = 1;
        while(num > 0 ){
            num = num - x;
            x = x + 2;
        }
        return num == 0;
    }
};