Question

I know the title is a bit messy, but I don’t know how to explain it better.

What I’m trying to do:

Using a graph found in a text file, find and print the shortest path (minimum amount of vertices) from vertex A to vertex B.

Note: using breadth-first search, not Dijkstra’s.

What I’ve got:

A working algorithm that applies BFS on the graph, but no good way of actually printing out the shortest path.

I’m having a hard time distinguishing a vertex in the shortest path from one that is simply run through the algorithm, but not in the shortest path.

For example: Find the shortest path between 0 and 4.
0 connects to 1,2 and 3. 1 connects to 4.
My path turns out to be [0,1,2,3,4] instead of [0,1,4].

I haven’t been able to find any threads asking the same question, or any walk-through of BFS that includes this, so I’m not sure if I’m making this out to be way harder than it is?

Edit: code for those who may be interested (not sure at all if I’m avoiding circles?)

Edit 2: Changed the way I store my path to a Stack.

public String findPath(int v, int w) {
    Queue<Integer> q = new LinkedList<Integer>();
    boolean[] visited = new boolean[g.numVertices()];

    q.add(v);
    Stack<Integer> path = new Stack<Integer>();
    while(q.peek() != null) {
        runBFS(q.poll(),w,visited,q,path);
    }
    return path.toString();
} 

private void runBFS(int v, int w, boolean[] visited, Queue<Integer> q, Stack<Integer> path) {
    if(visited[v]) {
    }
    else if(v == w) {
        path.add(v);
        q.clear();
    }
    else {
        path.add(v);
        visited[v] = true;
        VertexIterator vi = g.adjacentVertices(v);
        while(vi.hasNext()) {
                q.add(vi.next());
        }
    }
}

Some explanation of variables and methods:

v = vertex of origin

w = target vertex

g = graph

vi = a normal iterator that iterates over the neighbours of v

Thanks for reading!

Answer 1

You will have to have specific path field for each vertex. That way you can keep track of the paths you’ve chosen, hence the short path found. I will use an String array, just like you used the Boolean array for storing visited vertices.

public String findPath(int v, int w) {
    Queue<Integer> q = new LinkedList<Integer>();
    boolean[] visited = new boolean[g.numVertices()];
    String[] pathTo = new String[g.numVertices()];

    q.add(v);
    pathTo[v] = v+" ";
    while(q.peek() != null) {
        if(runBFS(q.poll(),w,visited,q,pathTo))
        break;
    }
    return pathTo[w];
}

private boolean runBFS(int v, int w, boolean[] visited, Queue<Integer> q, String[] pathTo) {
    if(visited[v]) {
    }
    else if(v == w)
        return true; 
    }
    else {
        visited[v] = true;
        VertexIterator vi = g.adjacentVertices(v);
        while(vi.hasNext()) {
            int nextVertex = vi.next();
            pathTo[nextVertex] = pathTo[v] + nextVertex + " ";
            q.add(nextVertex);
        }
    }
    return false;
}