using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        /*

             1             9

            /             /

           2            10

          / \           /

         4   3         11

        / \

       5   7

      /

     6

        */

        // Adjency List to describe the above structure

        // This would need to be created by DFS / BFS

        var adjacencyList = new Dictionary<int, int[]>();

        adjacencyList.Add(1, new int[]{2, 3, 4, 5, 6, 7});

        adjacencyList.Add(2, new int[]{4, 5, 6, 7});

        adjacencyList.Add(3, new int[]{});

        adjacencyList.Add(4, new int[]{5, 6, 7});

        adjacencyList.Add(5, new int[]{6});

        adjacencyList.Add(6, new int[]{});

        adjacencyList.Add(7, new int[]{});

        adjacencyList.Add(9, new int[]{10, 11});

        adjacencyList.Add(10, new int[]{11});

        adjacencyList.Add(11, new int[]{});

        var baseNodes = FindBaseNodes(adjacencyList);

        foreach (var baseNode in baseNodes)

        {

            Console.WriteLine(baseNode);

        }

    }

    public static List<int> FindBaseNodes(Dictionary<int, int[]> adjacencyList) 

    {

        var baseNodes = new List<int>();

        // Loops keys (every node in the collection)

        foreach (var node in adjacencyList.Keys)

        {

            var foundAsChild = false;

            // Loops keys again to search if original node exists as a child of any others

            foreach (var node1 in adjacencyList.Keys)

            {

                // Skip self

                if (node == node1)

                {

                    continue;

                }

                // Search other nodes children

                foreach (var childNode in adjacencyList[node1])

                {           

                    if (childNode == node)

                    {

                        // This node is not a base node

                        foundAsChild = true;    

                    }

                }

            }

            // Add node as a base node

            if (!foundAsChild)

            {

                baseNodes.Add(node);

            }

        }

        return baseNodes;

    }

}