4. LeetCode 367. 有效的完全平方数

 代码：

class Solution {
public:
    bool isPerfectSquare(int num) {
        long long a = (long long)num;
        long long left = 0;
        long long right = a;
        while (left <= right) {
            long long mid = left + ((right - left) >> 1);
            if ((mid * mid) < a) {
                left = mid + 1;
            }
            else if ((mid * mid) > a) {
                right = mid -1;
            } 
            else {
                return true;
            }
    }
    return false;
    }
};