Leetcode 269 Alien Dictionary - debug the code, work on coding, runs ok with test case: "wrt", "wrf", "er", "ett", "rftt"

Leetcode269AlienDictionary_P2.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)
        {
            string[] words = { "wrt", "wrf", "er", "ett", "rftt" };

            string result = alienOrder(words); 
        }

        /*
         * source code from blog:
         * http://www.cnblogs.com/yrbbest/p/5023584.html
         * 
         * Topological sorting - Kahn's Algorithm
         */
        public static 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) {
                set.UnionWith(word.ToList());                
            }
        
            int[] inDegree = new int[26];

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

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

            // Topological sort - starting from nodes with indegree value 0 
            // put all those nodes into queue first. 
            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();

            /*
             * keep updating indgree value based on the queue's operation 
             * once the node's indegree value's 0, push node to the queue. 
             * That is how it works - continue to output chars
             */
            while (queue.Count > 0) {
                char c = queue.Peek();
                sb.Append(c);
                queue.Dequeue(); // bug001 - Do not forget to dequeue 

                if (graph.ContainsKey(c)) {
                    foreach (char runner in graph[c]) {
                        int index = runner - 'a';
                        inDegree[index]--;

                        if (inDegree[index] == 0)
                        {
                            queue.Enqueue(runner);
                        }
                    }
                }
            }

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