Single Number II

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?

import java.util.HashMap;
public class Solution {
    public int singleNumber(int[] A) {
        HashMap<Integer, Integer> hashMap = new HashMap<Integer, Integer>();
        
        for(int i = 0; i < A.length; i++){
            Integer temp = hashMap.get(A[i]);
            if(null == temp)
                hashMap.put(A[i], 1);
            else if(1 == temp)
                hashMap.put(A[i], 2);
            else
                hashMap.put(A[i], 3);
        }
        for(int i = 0; i < A.length; i++){
            Integer temp = hashMap.get(A[i]);
            if(temp == 1)
                return A[i];
        }
        return 0;
    }
}