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?

class Solution {
public:
     int singleNumber(int A[], int n)
{
    sort(A,A+n);
    for(int i=0;i<n;i=i+3)
    {
        if(A[i]!=A[i+2])
            return A[i];
    }
}
};