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?

// I. using extra memory

class Solution {

public:
    int singleNumber(int A[], int n) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        map<int, int> table;
        table.clear();
        map<int, int>::iterator iter;
        for (int i=0;i<n;++i){
            iter = table.find(A[i]);
            if (iter==table.end())
                table[A[i]] = 1;
            else
                table[A[i]]++;
        }
        
        for (iter = table.begin();iter!=table.end();++iter){
            if ((iter->second)%3==1)
                return iter->first;
        }
    }
};

// without using extra space

class Solution {

public:
    int singleNumber(int A[], int n) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
       
        int n0, n1, n2;
        n1 = 0; n2 = 0;
        
        for (int i=0;i<n;i++){
            n0 = ~(n1|n2);
            n2 = (n2 & ~A[i]) | (n1 & A[i]);
            n1 = (n1 & ~A[i]) | (n0 & A[i]);
        }
        return n1;
    }
};