using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var graph=new Graph(4);

        var words = new string []{"baa", "abcd", "abca", "cab", "cad"};

        for(int i=0;i<words.Length -1; i++){

            int loopEnd = words[i].Length < words[i+1].Length ? words[i].Length : words[i+1].Length;

            for(int j=0;j<loopEnd;j++){

                if(words[i][j] != words[i+1][j]){

                    graph.AddEdge(words[i][j]-'a',words[i+1][j]-'a' );

                    break;

                }

            }

        }

        graph.TopologicalSort();

    }

}

​

public class Graph{

    public List<int>[] Vertices{get;set;}

    private int _verticeCount;

    public int VerticeCount{get{return _verticeCount;}}

    public Graph(int vertices){

        _verticeCount = vertices;

        Vertices= new List<int>[VerticeCount];

    }

    //only directed edges

    public void AddEdge(int u, int v){

        if(u<0 || v<0 || u>=VerticeCount || v>=VerticeCount){

            //throw error

            return;

        }

        if(Vertices[u] ==null){

            Vertices[u] = new List<int>();

        }

        Vertices[u].Add(v);

    }

    public void TopologicalSort(int vertex, bool[] visited, Stack<int> result){

        visited[vertex] =true;

        if(Vertices[vertex] !=null){

            foreach(int edge in Vertices[vertex]){

                if(!visited[edge]){ 

                    TopologicalSort(edge,visited,result);

                }

            }

        }

        result.Push(vertex);

    }

    public void TopologicalSort(){

        bool[] visited=new bool[VerticeCount];

        Stack<int> result = new Stack<int>();

        for(int i=0;i<VerticeCount; i++){

            if(!visited[i]){

                TopologicalSort(i,visited,result);

            }

        }

        Console.WriteLine();

        while(result.Count>0){

            Console.Write("=>{0}", (char)(result.Pop() + 'a'));

        }

    }   

}

​

​