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

Analysis:

This problem is the same as the problem of leetcode 69: https://codebysteven.wordpress.com/2016/09/08/leetcode-69-sqrtx/

Basically we can just change the return value from double to be boolean.

Time complexity: O(logn)

Space complexity: O(1)

Below is the code:

public class Solution {
    public boolean isPerfectSquare(int num) {
        // O(logn) binary search
        
        // Corner case
        if (num == 0 || num == 1) {
            return true;
        }
        
        long start = 1;
        long end = num;
        
        while (start + 1 < end) {
            long mid = start + (end - start) / 2;
            
            if (mid * mid == num) {
                return true;
            } else if (mid * mid < num) {
                start = mid;
            } else {
                end = mid;
            }
        }
        
        return false;
    }
}