Question

I am trying to discover all nodes reachable from a start node.

It seems like the base case is never reached, I am not sure why.

My code:

class Vertex {
    public final String name;
    public int found;
    public Edge[] adjacencies;
    public Vertex(String argName) { name = argName; found = 0;}
}

class Edge {
    public final Vertex target;
    public Edge(Vertex argTarget, double argWeight) { target = argTarget; weight = argWeight; }
}

ArrayList<Vertex> merge(ArrayList<Vertex> a, ArrayList<Vertex> b) {
    if(a == null) return b;
    if(b == null) return a;
    for(int x = 0; x < a.size(); x++)
        b.add(a.get(x));
        return b;
}

ArrayList<Vertex> span(Vertex b) {
    System.out.println("hello");
    if(b.found == 1) {
        return null;
    }
    ArrayList<Vertex> t = new ArrayList<Vertex>();
    t.add(b);
    b.found = 0;
    if(b.adjacencies == null) return t;
    for(int x = 0; x < b.adjacencies.length; x++) {
        ArrayList<Vertex> result = span(b.adjacencies[x].target);
        t = merge(t, result);
    }
    return t;
}

I am getting a stack-overflow error, it seems like it is stuck in recursion, but I am not sure why.

Note: This is the graph that I am running it on.

Vertex v0 = new Vertex("Redvile");
Vertex v1 = new Vertex("Blueville");
Vertex v2 = new Vertex("Greenville");
Vertex v3 = new Vertex("Orangeville");
Vertex v4 = new Vertex("Purpleville");

v0.adjacencies = new Edge[]{ new Edge(v1, 5), new Edge(v2, 10), new Edge(v3, 8) };
v1.adjacencies = new Edge[]{ new Edge(v0, 5), new Edge(v2, 3), new Edge(v4, 7) };
v2.adjacencies = new Edge[]{ new Edge(v0, 10), new Edge(v1, 3) };
v3.adjacencies = new Edge[]{ new Edge(v0, 8), new Edge(v4, 2) };
v4.adjacencies = new Edge[]{ new Edge(v1, 7), new Edge(v3, 2) };

Answer 1

You never set found to 1, you only set it to 0 while you probably meant 1.
Also, use a boolean for found.
Also, use addAll for ArrayLists, and return Collections.emptyList, instead of null.