using System;

public class Program

{

    public static int SingleNumber(int[] nums)

    {

        int[] nums2 = new int[nums.Length];

​

        if (nums.Length == 1)

        {

            return nums[0];

        }

​

        for (int i = 0; i < nums.Length; i++)

        {

            if (Array.IndexOf(nums2, nums[i]) < 0) // //if (!nums2.Contains(nums[i]))

            {

                nums2[i] = nums[i];

            }

            else

            {

                var x = Array.IndexOf(nums2, nums[i]);

                nums[x] = 0;

                nums[i] = 0;

            }

        }

​

        var index = Array.FindIndex(nums, value => value != 0);

        return nums[index];

    }

    public static void Main()

    {

        int[] nums = new int[] { 1, 2, 3, 3, 1};

        int result = SingleNumber(nums);

        Console.WriteLine(result);

    }

}