Add solutions for CodeWars challenges

CodeWars/Array-diff.cs

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+using System;
+using System.Collections;
+using System.Collections.Generic;
+
+public class Kata
+{
+    public static int[] ArrayDiff(int[] a, int[] b)
+    {
+        var hashSet = new HashSet<int>(b);
+        var response = new List<int>();
+
+        foreach (var x in a)
+        {
+            if (!hashSet.Contains(x))
+            {
+                response.Add(x);
+            }
+        }
+
+        return response.ToArray();
+    }
+}

CodeWars/LinkedLists-InsertNthNode.cs

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+using System;
+
+public partial class Node
+{
+    public int Data;
+    public Node Next;
+
+    public Node(int data)
+    {
+        this.Data = data;
+        this.Next = null;
+    }
+
+    public static Node InsertNth(Node head, int index, int data)
+    {
+
+        if (index < 0)
+            throw new ArgumentOutOfRangeException(nameof(index), "Index is out of the range of the list.");
+
+        var newNode = new Node(data);
+
+        if (index == 0)
+        {
+            newNode.Next = head;
+            return newNode;
+        }
+
+        var current = head;
+        int count = 0;
+
+        while (current != null && count < index - 1)
+        {
+            current = current.Next;
+            count++;
+        }
+
+        if (current == null)
+        {
+            throw new ArgumentOutOfRangeException(nameof(index), "Index is out of the range of the list.");
+        }
+
+        newNode.Next = current.Next;
+        current.Next = newNode;
+
+        return head;
+
+    }
+}

CodeWars/Parse-a-linked-list-from-a-string.cs

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+using System;
+
+public static class Kata
+{
+    public static Node Parse(string nodes)
+    {
+        Node head = null;
+        Node prev = null;
+        var array = nodes.Split(" -> ");
+
+        foreach (var current in array)
+        {
+            if (current.Trim() == "null")
+                break;
+
+            var value = int.Parse(current.Trim());
+
+            var newNode = new Node(value);
+
+            if (head == null)
+            {
+                head = newNode;
+            }
+            else
+            {
+                prev.Next = newNode;
+            }
+
+            prev = newNode;
+        }
+
+        return head;
+    }
+}