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;
}
}