Question

So I wrote a Graph class and I can’t seem to do a depth first search on it properly depending on the sequencing of nodes. Here’s what I mean:

If my graph looks like this:

A-B-D
|/
C

The DFS returns: “ABC”

But when it looks like this:

A-B
| |
D C
|
E

It will print ABCDE correctly.

The problem I’ve found lies in my getUnvisitedAdjacentNode() function. Here is the function:

    public int getUnvisitedAdjacentNode(int n) {
    for (int i = 0; i < this.nodeList.size(); i++) {
        if (this.edges[n][i] == 1 && this.nodeList.get(i).wasVisited == false) {
            return i;
        }
    }
    return -1;
}

The problem, I’ve found is because it goes in “order” (just a for loop) , it will never get traverse D in the first situation because B gets visited and after C gets visited, B simply get’s popped off of the stack. Maybe this isn’t the problem.

Here’s the code for my actual DFS traversal.

   public void depthFirstTraverse() {
    Stack<Node> stack = new Stack<Node>();

    nodeList.get(0).wasVisited = true;
    System.out.println(nodeList.get(0).item);
    stack.push(nodeList.get(0));

    while (!stack.isEmpty()) {
        int nextNode = this.getUnvisitedAdjacentNode(stack.peek().index);

        if (nextNode == -1) {
            stack.pop();
        } else {
            nodeList.get(nextNode).wasVisited = true;
            System.out.println(nodeList.get(nextNode).item);
            stack.push(nodeList.get(nextNode));
        }
    }
    for (int i = 0; i < nodeList.size(); i++) {
        nodeList.get(i).wasVisited = false;
    }
}

Answer 1

Fortunately I found my own mistake, the code above is all correct, except it was in the code that I hadn’t pasted.

In case anybody cares, the problem lied in the fact that I completely disregarded the fact that ArrayLists have an “IndexOf()” method (stupid, I know) and decided to hack my own “index” field into my Node class. When dealing with my own indices, I had a minor bug which screwed up the traversal.

So the old line in my DFS algorithm looks like this:

int nextNode = this.getUnvisitedAdjacentNode(stack.peek().index);

But it should be:

int nextNode = this.getUnvisitedAdjacentNode(this.nodeList.indexOf(stack.peek()));