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) {
        
        unordered_map<int, int> singleMap;
        for(int i = 0; i < n; i++)
        {
            if(singleMap.find(A[i]) != singleMap.end())
                singleMap[ A[i] ]++;
            else
                singleMap[ A[i] ] = 1;
        }
        
        for(unordered_map<int, int>::iterator it = singleMap.begin(); it != singleMap.end(); it++)
        {
            if(it->second != 3)
                return it->first;
        }
    }
};