Find duplicates in int array with O(n) using Hashtable

FindDuplicates.cs

//https://leetcode.com/problems/find-all-duplicates-in-an-array/

using System;
using System.Collections;
using System.Collections.Generic;

public class Solution {
    public IList<int> FindDuplicates(int[] nums) {
        
        if (nums is null)
            return null;
        
        List<int> duplicates = new List<int>();
        
        if (nums.Length < 2)
            return duplicates;
        
        Hashtable hashTable = new Hashtable();
        hashTable.Add(nums[0], nums[0]);
    
        for(int x = 1, elem = nums[1]; x < nums.Length; ++x)
        {
            elem = nums[x];
            
            if (hashTable.ContainsKey(elem))
                duplicates.Add(elem);
            else
                hashTable.Add(elem, elem);
        }

        return duplicates;
    }
}

www.leetcode.com/problems/find-all-duplicates-in-an-array/

O(n) complexity