Single Number II <LeetCode>

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) {
        if(n==1)  return A[0];
        sort(A,A+n);
        int i=0;
        while(A[i]==A[i+2]&&1!=n-1)
        {
            i=i+3;
        }
        return A[i];
    }
};