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：

public class Solution {
    public boolean isPerfectSquare(int num) {
        int left = 1;
        int right = num;
        while(left<=right){
            long mid = left+(right-left)/2;
            if(mid*mid==num) return true;
            else if(mid*mid<num) left = (int)mid+1;
            else right = (int)mid-1;
        }
        return false;
    }
}

有一个效率不高的做法，代码如下：

public boolean isPerfectSquare(int num) {
     int i = 1;
     while (num > 0) {
         num -= i;
         i += 2;
     }
     return num == 0;
 }