367. Valid Perfect Square

Problem:

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:
Be careful with int/int, if num is 5, 5/2=2.
Solution:
To include 1, use start <= end as while condition. Easy but not trivial.

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