Leetcode 269 - Alien Dictionary - source code from blog: http://www.cnblogs.com/yrbbest/p/5023584.html -> C# code

Leetcode269AlienDictionary_P1.cs

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

namespace _269AlienDictionary
{
    class Program
    {
        static void Main(string[] args)
        {
        }

        /*
         * source code from blog:
         * http://www.cnblogs.com/yrbbest/p/5023584.html
         * 
         * Topological sorting - Kahn's Algorithm
         */
        public String alienOrder(String[] words) {   
            if(words == null || words.Length == 0) {
                return "";
            }

            Dictionary<char, HashSet<char>> graph = new Dictionary<char, HashSet<char>>();

            HashSet<char> set = new HashSet<char>();

            foreach (string word in words) {
                for (int i = 0; i < word.Length; i++) {
                    set.Add(word[i]);
                }
            }
        
            int[] inDegree = new int[26];

            for (int k = 1; k < words.Length; k++) {
                
                String preStr = words[k - 1];
                String curStr = words[k];

                for (int i = 0; i < Math.Min(preStr.Length, curStr.Length); i++) {
                    char preChar = preStr[i];
                    char curChar = curStr[i];

                    if (preChar != curChar) {
                        if (!graph.ContainsKey(preChar)) {
                            graph.Add(preChar, new HashSet<char>());
                        }

                        if (!graph[preChar].Contains(curChar)) {
                            inDegree[curChar - 'a']++;
                        }  
                  
                        graph[preChar].Add(curChar);
                        break;
                    }
                }
            }

            Queue<char> queue = new Queue<char>();
            for (int i = 0; i < inDegree.Length; i++) {
                if (inDegree[i] == 0) {
                    char c = (char)('a' + i);
                    if (set.Contains(c)) {
                        queue.Enqueue(c);    
                    }
                }
            }

            StringBuilder sb = new StringBuilder();

            while (queue.Count > 0) {
                char c = queue.Peek();
                sb.Append(c);

                if (graph.ContainsKey(c)) {
                    foreach (char l in graph[c]) {
                        inDegree[l - 'a']--;
                        if (inDegree[l - 'a'] == 0) {
                            queue.Enqueue(l);
                        }
                    }
                }
            }

            return sb.Length != set.Count ? "" : sb.ToString();
       }
    }
}