using System;

using System.Collections.Generic;

using System.Linq;

​

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 Stack<Node>();

        while(possibleStart.Count > 0)

        {

            var start = possibleStart.FirstOrDefault();

            possibleStart.Remove(start);

            var queue = new Queue<Node>();

            start.VisitStatus = VisitStatus.Visiting;

            queue.Enqueue(start);

            while(queue.Count > 0)

            {

                var current = queue.Dequeue();

                foreach(var ad in current.Adjacent)

                {

                    if(ad.VisitStatus == VisitStatus.Visiting)

                        continue;

                    ad.VisitStatus = VisitStatus.Visiting;

                    queue.Enqueue(ad);

                }

                current.VisitStatus = VisitStatus.Visited;

                result.Push(current);

            }

        }

        if(result.Count() != projects.Count()) 

            throw new ApplicationException("no possible order");

        while(result.Count > 0)

        {

            Console.Write(result.Pop().Character + (result.Any() ? "," : ""));

        }

    }

    private class Node

    {

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

        public char Character;

        public VisitStatus VisitStatus;

    }

    private enum VisitStatus

    {

        Unvisited,

        Visiting,

        Visited 

    }

}

​