using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        /*

        *   You are given a list of projects and a list of dependencies (which is a list 

        *   of pairs of projects, where the second project is dependent on the first project). 

        *   All of a project's dependencies must be built before the project is. Find a build

        *   order that will allow the projects to be built. If there is no valid build order, 

        *   return an error.

        *   input: 

        *       projects: a,b,c,d,e,f

        *       dependencies: (a,d),(f,b),(b,d),(f,a),(d,c)

        *   output:

        *       f,e,a,b,d,c

        */

        var projects = new char[] {'a','b','c','d','e','f'};

        var dependencies = new char[,] {{'a','f','b','f','d'},

                                        {'d','b','d','a','c'}};

        var nodes = new Dictionary<char, Node>();

        var possibleStart = new HashSet<Node>();

        foreach(var project in projects)

        {

            var node = new Node {Character = project};

            nodes.Add(project, node);

            possibleStart.Add(node);

        }

        for(var i = 0; i < dependencies.GetLength(1); i++)

        {

            nodes[dependencies[1,i]].Adjacent.Add(nodes[dependencies[0,i]]);

            possibleStart.Remove(nodes[dependencies[0,i]]);

        }

        var result = new List<char>();

        foreach(var start in possibleStart)

            GetRoute(start, result);

        Console.WriteLine(String.Join(",", result));

    }

    static void GetRoute(Node node, List<char> list)

    {

        node.VisitStatus = VisitStatus.Visiting;

        foreach(var ad in node.Adjacent)

        {

            if(ad.VisitStatus == VisitStatus.Visiting)

                throw new ApplicationException("no possible order");

            if(ad.VisitStatus == VisitStatus.Visited)

                continue;

            GetRoute(ad, list);

        }

        node.VisitStatus = VisitStatus.Visited;

        list.Add(node.Character);

    }

    private class Node

    {

        public List<Node> Adjacent = new List<Node>();

        public char Character;

        public VisitStatus VisitStatus;

    }

    private enum VisitStatus

    {

        Unvisited,

        Visiting,

        Visited 

    }

}

​