Leetcode 126 - word ladder - http://www.programcreek.com/2014/06/leetcode-word-ladder-ii-java/ with a test case, code works

Leetcode126_wordLadder_geeksblog.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace _126LadderLength_WarmupV
{
    class WordNode
    {
        public String word;
        public int numSteps;
        public WordNode pre;

        public WordNode(String word, int numSteps, WordNode pre)
        {
            this.word = word;
            this.numSteps = numSteps;
            this.pre = pre;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            string beginWord = "hit";
            string endWord = "cog";
            string[] arr = new string[5] { "hot", "dot", "dog", "lot", "log" };
            HashSet<string> wordList = new HashSet<string>(arr);

            List<List<string>> result = findLadders(beginWord, endWord, wordList);


        }

        /*
         * http://www.programcreek.com/2014/06/leetcode-word-ladder-ii-java/
         * 
         * From Java Code to C# code
         * 
         */
        public static List<List<String>> findLadders(String start, String end, ISet<String> dict)
        {
            List<List<String>> result = new List<List<String>>();

            LinkedList<WordNode> queue = new LinkedList<WordNode>();
            queue.AddLast(new WordNode(start, 1, null));

            dict.Add(end);  // add end word into HashSet

            int minStep = 0;

            HashSet<String> visited   = new HashSet<String>();
            HashSet<String> unvisited = new HashSet<string>(dict);            

            int preNumSteps = 0;

            while (queue.Count > 0)
            {
                // get used to practice using LinkedList in C# (double linked list), 
                // Java LinkedList is more popular
                WordNode top = queue.First();
                queue.RemoveFirst(); 

                String word = top.word;
                int currNumSteps = top.numSteps;

                if (word.CompareTo(end) == 0)
                {
                    if (minStep == 0)
                    {
                        minStep = top.numSteps;
                    }

                    if (top.numSteps == minStep && minStep != 0)
                    {
                        //nothing
                        List<String> t = new List<String>();
                        t.Add(top.word);
                        while (top.pre != null)
                        {
                            // use List<string> as an array, iterator - 0
                            t.Add(top.pre.word);   

                            top = top.pre;
                        }

                        result.Add(t);
                        continue;
                    }
                }

                if (preNumSteps < currNumSteps)
                {
                    unvisited.ExceptWith(visited);  // first time, use HashSet ExceptWith api
                }

                preNumSteps = currNumSteps;

                char[] arr = word.ToCharArray();
                for (int i = 0; i < arr.Length; i++)
                {
                    for (char c = 'a'; c <= 'z'; c++)
                    {
                        char temp = arr[i];
                        if (arr[i] != c)
                        {
                            arr[i] = c;
                        }

                        String newWord = new String(arr);
                        if (unvisited.Contains(newWord))
                        {
                            queue.AddLast(new WordNode(newWord, top.numSteps + 1, top));
                            visited.Add(newWord);
                        }

                        arr[i] = temp;
                    }
                }


            }

            return result;
        }
    }
}