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

使用二分查找尋找input的開方值；

使用long long防止溢出。

我的代碼如下：

class Solution {
 
public:
    bool isPerfectSquare(int num) {
        if (num == 1) return true;
        long long left = 1, mid = (num + 1) / 2, right = num;
        while (left <= right) {
            if (mid * mid == num) return true;
            if (mid * mid > num) right = mid - 1;
            else left = mid + 1
 
;
            mid = (left + right) / 2;
        }
        return false;
    }
};

[leetcode] 367. Valid Perfect Square