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

題目標簽：Math

　　題目給了我們一個 num， 讓我們判斷它 是不是 perfect square。

　　可以利用binary search來找，需要註意overflow。

Java Solution:

Runtime beats 41.41%

完成日期：06/16/2017

關鍵詞：binary search

關鍵點：利用binary search 來找到 mid * mid == num

 1 class Solution 
 2 {
 3     public boolean isPerfectSquare(int num) 
 4     {
 5         if(num <= 0)
 6             return false;
 
 7         
 8         int left = 0;
 9         int right = num;
10         
11         while(left <= right)
12         {
13             int mid = left + (right - left) / 2;
14             
15             int sq = mid * mid;
16             // check overflow here
17             if(sq != 0 && sq / mid != mid)
 
18             {
19                 right = mid - 1;
20                 continue;
21             }
22             
23 
24             if(sq == num)
25                 return true;
26             else if(sq > num) {
27                 right = mid - 1;
28             }
29             else
30             {
31                 left = mid + 1;
32             }
33             
34         }
35         
36         return false;
37     }
38 }

參考資料：n/a

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LeetCode 367. Valid Perfect Square