【easy】367.ValidPerfectSquare判断是不是平方数

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 <0) return false;
        if (num == 1) return true;
        int left = 0;       //注意！
        int right = num/2;  //注意！
        long mid;
        long val;
        while (left <= right){
            mid = (left + right)/2;
            val = mid * mid;
            if (val == num) return true;
            else if (val > num) right = mid - 1;
            else left = mid + 1;
        }
        return false;
    }     
        
};