class Solution {

public:

    bool isPerfectSquare(int num) {

        /*

        //方法一：蜜汁超时……

        if (num < 0) return false;

        if (num == 1) return true;

        for (int i=1;i*i<=num;i++){

            if (i*i==num)

                return true;

        }

        return false;

    }*/

        /*

        //方法二：是对的！

        if(num < 0) return false;

        if(num == 1) return true;

        for(int i = 1; i<= num/i;i++){

              if(i*i == num) return true;

        }

        return false;

    } */

        //方法三：值得学习的【二分查找】

        if (num < ) return false;

        if (num == ) return true;

        int left = ;       //注意！

        int right = num/;  //注意！

        long mid;

        long val;

        while (left <= right){

            mid = (left + right)/;

            val = mid * mid;

            if (val == num) return true;

            else if (val > num) right = mid - ;

            else left = mid + ;

        }

        return false;

    }     

};