367. Valid Perfect Square

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

Credits:Special thanks to @elmirap for adding this problem and creating all test cases.

Solution

public class Solution {
  public boolean isPerfectSquare(int num) {
        if (num < 0) return false;

        long start = 1;
        long end = num;

        while (start + 1 < end) {
            long middle = start + (end - start) / 2;
            long square = middle * middle;
            if (square == num) {
                return true;
            } else if (square < num) {
                start = middle;
            } else {
                end = middle;
            }
        }

        if (start * start == num) {
            return true;
        }

        if (end * end == num) {
            return true;
        }

        return false;
    }
}