成语接龙，哈希表神马的最给力了。

using System;
using System.Collections.Generic;
using System.IO;
using System.Text;

namespace ConsoleApplication1
{
    public class Idiom
    {
        public static Dictionary<Char, List<int>> Dict = new Dictionary<char, List<int>>();
        public static List<Idiom> IdiomList = new List<Idiom>();
        static Idiom()
        {
           string[] Arr= File.ReadAllLines(Environment.CurrentDirectory + @"\idiom.txt",Encoding.UTF8);
           for (int i = 0; i < Arr.Length; i++)
           {
               Idiom I = new Idiom(i, Arr[i]);
               IdiomList.Add(I);
               if (!Dict.ContainsKey(I.Head))
               {
                   Dict.Add(I.Head, new List<int>());
               }
               Dict[I.Head].Add(I.ID);
           }
        }
        public Idiom(int id,string word )
        {
            ID = id;
            Word = word;
            char[] C = word.ToCharArray();
            Head = C[0];
            Tail = C[word.Length - 1];
        }
        public string Word;
        public int ID;
        public char Tail;
        public char Head;
    }
}

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

namespace ConsoleApplication1
{
    public class Dragon
    {
        public Dragon(Idiom I )
        {
            Body.Add(I.ID);
            Tail = I.Tail;
        }
        public Dragon(Dragon D, Idiom  I)
        {
            Body = new HashSet<int>(D.Body);
            Body.Add(I.ID);
            Tail = I.Tail;
        }
        public HashSet<int> Body = new HashSet<int>();
        public Char Tail{get;set;} 
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Diagnostics;
using System.IO;

namespace ConsoleApplication1
{
    class Program
    {

        static Stopwatch SW = new Stopwatch();
        static bool Finish = true;        
        static Dragon maxDragon = null;
        static int maxLength = 1;
        static void Main(string[] args)
        {
            SW.Start();
            char input = '三';
            if (Idiom.Dict.ContainsKey(input))
            {
                List<int> First = Idiom.Dict[input];
                foreach (int i in First)
                {
                    Dragon D = new Dragon(Idiom.IdiomList[i]);
                    if (Finish) { maxDragon = D; Finish = false; }
                    DFS(D);
                }
            }
            else
            {
                Console.WriteLine("输入的词不存在");
            }

            Console.Read();
        }

        private static void DFS(Dragon D)
        {
            
            if (SW.ElapsedMilliseconds > 1000 && !Finish)
            {
                SW.Stop();
                
                if (maxDragon.Body.Count == maxLength)
                {
                    foreach (int i in maxDragon.Body)
                    {
                        Console.Write(Idiom.IdiomList[i].Word + ",");
                    }
                    Console.WriteLine(maxDragon.Body.Count);
                }
                
                Finish = true;
                return;
            }
            if (Finish) { return; } 
            if (Idiom.Dict.ContainsKey(D.Tail))
            {
                foreach (int i in Idiom.Dict[D.Tail])
                {
                    if (!D.Body.Contains(i))//无环
                    {
                        Dragon newDragon = new Dragon(D, Idiom.IdiomList[i]);
                        if (newDragon.Body.Count > maxDragon.Body.Count)
                        {
                            maxDragon = newDragon;
                            maxLength=newDragon.Body.Count;
                        }
                        DFS(newDragon);
                    }
                }
            }
        }
    }
}