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 = ;
         long end = num;

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