Leetcode: Single Number II

Given an array of integers, every element appears three times 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         int count = 1;
 5         int i;
 6         for (i = 1; i < A.length; i++) {
 7             if (A[i] == A[i-1]) {
 8                 count++;
 9             }
10             else {
11                 if (count != 3) return A[i-1];
12                 count = 1;
13             }
14         }
15         return A[i-1];
16     }
17 }