Preorder traversal through Adjacency List

Question

I am trying to convert my EdgeList into a Adjacency List and then preorder traverse through it. I am pretty sure the conversion to Adjacency List is working correctly, but I am having troubles with preorder traversing through it. I have tried to do it with DFS, but it gives me the wrong result.

Here are my edges:

{index1=0, index2=2}
{index1=3, index2=4}
{index1=1, index2=4}
{index1=0, index2=5}
{index1=2, index2=6}
{index1=1, index2=5}
{index1=2, index2=7}

And this is what the linked list looks like.

0 - 2 - 5
1 - 4 - 5
2 - 6 - 7
3 - 4

Or

linked[2, 5]
linked[4, 5]
linked[6, 7]
linked[4]

Now I want to preorder traverse through the graph, to get the result 0 - 2 - 6 - 7 - 5 - 1 - 4 - 3.

          0
      2       5
   6    7       1
                  4
                   3

I have tried using DFS in my AdjacencyListGraph, but this is the result I get 0 2 6 7 5 with this code:

public class AdjacencyListGraph {

    private int V;   // No. of vertices

    // Array  of lists for Adjacency List Representation
    private LinkedList<Integer> adj[];

    // Constructor
    AdjacencyListGraph(int v) {
        V = v;
        adj = new LinkedList[v];
        for (int i=0; i<v; ++i)
            adj[i] = new LinkedList();
    }

    //Function to add an edge into the graph
    void addEdge(int v, int w) {
        adj[v].add(w);  // Add w to v's list.
    }


    // A function used by DFS
    void DFSUtil(int v,boolean visited[])
    {
        // Mark the current node as visited and print it
        visited[v] = true;
        System.out.print(v+" ");

        // Recur for all the vertices adjacent to this vertex
        Iterator<Integer> i = adj[v].listIterator();
//        System.out.println(i.toString());
        while (i.hasNext())
        {
            int n = i.next();
            if (!visited[n])
                DFSUtil(n,visited);
        }
    }

    // The function to do DFS traversal. It uses recursive DFSUtil()
    void DFS(int v)
    {

        // Mark all the vertices as not visited(set as
        // false by default in java)
        boolean visited[] = new boolean[V];

        // Call the recursive helper function to print DFS traversal
        // starting from all vertices one by one

        DFSUtil(v, visited);
    }

    public static void main(String[] args) {
        AdjacencyListGraph graph = new AdjacencyListGraph(8);

        graph.addEdge(0, 2);
        graph.addEdge(0, 5);
        graph.addEdge(1, 4);
        graph.addEdge(1, 5);
        graph.addEdge(2, 6);
        graph.addEdge(2, 7);
        graph.addEdge(3, 4);

        graph.DFS(0);
    }
}

I have also tried using:

void DFS() 
{ 
    // Mark all the vertices as not visited(set as 
    // false by default in java) 
    boolean visited[] = new boolean[V]; 

    // Call the recursive helper function to print DFS traversal 
    // starting from all vertices one by one 
    for (int i=0; i<V; ++i) 
        if (visited[i] == false) 
            DFSUtil(i, visited); 
} 

Instead of the one mentioned above and it goes through all of the nodes, but it returns the result in wrong order.

What am I doing wrong and how or what should I use to get the desired result?

Answer 1

Try with this graph:

graph.addEdge(0, 2);
graph.addEdge(0, 5);
graph.addEdge(1, 4);
graph.addEdge(5, 1); // different direction than original
graph.addEdge(2, 6);
graph.addEdge(2, 7);
graph.addEdge(4, 3); // different direction than original

Or, make a slight modification in addEdge method in your program:

//Function to add an edge into the graph
void addEdge(int v, int w) {
    adj[v].add(w);  // Add w to v's list.
    adj[w].add(v);  // Add v to w's list.
}