367. Valid Perfect Square

Q:

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

A:

********** 在所有的可能的平方根里，进行二分查找**********

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

Mistakes:

if we declare:

int mid; 

mid * mid   would be an int,  but can overflow, thus need long.