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?

public class Solution {
    public int singleNumber(int[] A) {
            HashMap<Integer, Integer> map = new HashMap<>();
            int singnum=0;
            for (int i = 0; i < A.length; i++) {
                if (map.containsKey(A[i])) {
                    map.put(A[i], map.get(A[i])+1);
                }else {
                    map.put(A[i],1);
                }
                if (map.get(A[i])==3) {
                    map.remove(A[i]);
                }
            }
            for (Integer i : map.keySet()) {
                singnum=i;
            }
            return singnum;
    }
}