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

本题开始做的时候并没有什么思路，后来看了tag的提示知道了可以用binary search来做，本题需要注意的是越界问题，代码如下：

 1 public class Solution {
 2     public boolean isPerfectSquare(int num) {
 3         int left = 1;
 4         int right = num;
 5         while(left<=right){
 6             long mid = left+(right-left)/2;
 7             if(mid*mid==num) return true;
 8             else if(mid*mid<num) left = (int)mid+1;
 9             else right = (int)mid-1;
10         }
11         return false;
12     }
13 }

有一个效率不高的做法，代码如下：

1 public boolean isPerfectSquare(int num) {
2      int i = 1;
3      while (num > 0) {
4          num -= i;
5          i += 2;
6      }
7      return num == 0;
8  }