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

分析：
二分。注意溢出！

public class Solution {
    public boolean isPerfectSquare(int num) {
        long begin = 1;
        long end = num;

        while (begin <= end) {
            long mid = (end + begin) / 2;
            if (mid * mid == num) {
                return true;
            } else if (mid * mid < num) {
                begin = mid + 1;
            } else {
                end = mid - 1;
            }
        }
        return false;
    }
}