using System.Collections.Generic;
public static void Main()
var result = FindDuplicates(new int[]{4,3,2,7,8,2,3,1});
Console.WriteLine(string.Join(",", result.ToArray()));
static IList<int> FindDuplicates(int[] nums) {
IList<int> result = new List<int>();
for(int i = 0; i < nums.Length; i++)
var index = Math.Abs(nums[i]) - 1;