367 valid perfect square numbers

367. 有效的完全平方数

cpp：

正确代码：

class Solution {
public:
    bool isPerfectSquare(int num)
    {
        if (num < 2) //小于2时肯定为完全平方数
        {
            return true;
        }
        
        int left = 1;
        int right = num / 2;
        long long square = 0; //直接使用long long，避免整数除法带来的精度误差

        while (left <= right)
        {
            int mid = left + (right - left) / 2; //防止溢出
            square = (long long) mid * mid; //式子里也要带long long才行
            if (square > num)
            {
                right = mid - 1;
            }
            else if (square < num)
            {
                left = mid + 1;
            }
            else
            {
                return true;
            }
        }

        return false;
    }
};

错误代码：

class Solution {
public:
    bool isPerfectSquare(int num)
    {
        int square = 0;
        if (num < 2)
        {
            square = num;
        }
        
        int left = 1;
        int right = num / 2;
        while (left <= right)
        {
            int mid = left + (right - left) / 2;
            if (mid > num / mid)
            {
                right = mid - 1;
            }
            else
            {
                left = mid + 1;
                square = mid;
            }
        }

        if (square == num / square)
        {
            return true;
        }
        
        return false;
    }
};