Given an array of integers, every element appearsthreetimes except for one, which appears exactly once. Find that single one.

Note:
Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

class Solution {
    public int singleNumber(int[] nums) {
        Set<Integer> set = new HashSet<>();

        for (int num : nums){
            set.add(num);
        }

        long sum1 = 0;
        for (int num : set){
            sum1 += num;
        }

        long sum2 = 0;
        for (int num : nums){
            sum2 += num;
        }

        return (int)((sum1 * 3 - sum2) / 2);
    }
}