LeetCode Single Number

Given an array of integers, every element appears twice 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) {
int singlenum = 0;
HashSet<Integer> set = new HashSet<>();
for (int i = 0; i < A.length; i++) {
if (set.contains(A[i])) {
set.remove(A[i]);
}else {
set.add(A[i]);
}
}
for (Integer integer : set) {
singlenum=integer;
}
return singlenum;
}
}