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;
    }
}