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
Output: true

Example 2:

Input: 14
Output: false

class Solution {
    public boolean isPerfectSquare(int num) {
        if(num == 1) return true;
        for(int i = 1; i <= num/2; i++){
            if(i*i == num) return true;
        }
        return false;
    }
}

brute force O(n/2)，慢

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

 芜湖