Leetcode: Single Number

Given an array of integers, every element appears twice except for one. Find that single one.

Note:
Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

 1 public class Solution {
 2     public int singleNumber(int[] A) {
 3         java.util.Arrays.sort(A);
 4         if (A.length == 1) return A[0];
 5         int i = 1;
 6         while (i < A.length) {
 7             if (A[i] != A[i-1]) return A[i-1];
 8             i += 2;
 9         }
10         return A[i-1];
11     }
12 }