题目：
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
Returns: True
Example 2:
Input: 14
Returns: False 思路：
分析规律：
1x1=1 1
2x2=4 1+3
3x3=9 1+3+5
4x4=25 1+3+5+7
nxn = 1+3+5+...(2xn-1)
代码：
class Solution {
public:
bool isPerfectSquare(int num) {
int i = 1;
while (num > 0) {
num -= i;
i += 2;
}
return num == 0;
}
};
