LeetCode 367. Valid Perfect Square

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

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题目标签：Math

　　题目给了我们一个 num， 让我们判断它 是不是 perfect square。

　　可以利用binary search来找，需要注意overflow。

Java Solution:

Runtime beats 41.41% 

完成日期：06/16/2017

关键词：binary search

关键点：利用binary search 来找到 mid * mid == num

class Solution 
{
    public boolean isPerfectSquare(int num) 
    {
        if(num <= 0)
            return false;
        
        int left = 0;
        int right = num;
        
        while(left <= right)
        {
            int mid = left + (right - left) / 2;
            
            int sq = mid * mid;
            // check overflow here
            if(sq != 0 && sq / mid != mid)
            {
                right = mid - 1;
                continue;
            }
            

            if(sq == num)
                return true;
            else if(sq > num) {
                right = mid - 1;
            }
            else
            {
                left = mid + 1;
            }
            
        }
        
        return false;
    }
}

参考资料：n/a

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