nthFromLastElement, C#

nthFromLastElement

//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Node {
      public int data;
      public Node child;
        
        public Node (int data, Node child ){
             data = data;
             child = child;
        }  
    }
    
    public class Program
    {
        public static void Main(string[] args)
        {
            Node current = new Node(1, null);
            for (int i = 2; i < 8; i++) {
                current = new Node(i, current);
            }
            Node head = current;
            // head = 7 -> 6 -> 5 -> 4 -> 3 -> 2 -> 1 -> (null)

Node current2 = new Node(4, null);
            for (int i = 3; i > 0; i--) {
                current2 = new Node(i, current2);
            }
            Node head2 = current2;
            // head2 = 1 -> 2 -> 3 -> 4 -> (null)
            //Node return = nthFromLast(head, 1);
            //Console.WriteLine(return.data);
            // nthFromLast(head, 1) should return 1.
            // nthFromLast(head, 5) should return 5.
            // nthFromLast(head2, 2) should return 3.
            // nthFromLast(head2, 4) should return 1.
            // nthFromLast(head2, 5) should return null.
            // nthFromLast(null, 1) should return null.
        }
        
        public static Node nthFromLast( Node head, int n)
        {
            if( head == null ) return null;
            Node p = head;
            Node q = head;
            for (int i=0; i<n; i++ ){
                if( q == null ) return null;
                q = head.child;
            }
            
            while ( q != null ) {
                q = q.child;
                p = p.child;
            }
            return p;
        }
    }
}


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