BFS search in 2D grid Java shortest path

Question

I've managed to come up with a working BFS algorithm that returns the entire traversal path, however I'm unsure of how to get the shortest path.

This is my code:

public ArrayList<Node> get_neighbours(Node node, Grid grid) {

        ArrayList<Node> neighbours = new ArrayList<>();

        int R = grid.getRl();
        int C = grid.getCl();

        int[] dr = new int[]{-1, +1, 0, 0};
        int[] dc = new int[]{0, 0, +1, -1};

        for (int i=0; i<4; i++) {
            int rr = node.getR() + dr[i];
            int cc = node.getC() + dc[i];

            if (rr < 0 || cc < 0) continue;
            if (rr >= R || cc >= C) continue;

            if (grid.getNodes()[rr][cc].getVal() == "V") continue;
            if (grid.getNodes()[rr][cc].getVal() == "X") continue;

            Node neighbour = grid.getNodes()[rr][cc];
            neighbours.add(neighbour);
        }

        return neighbours;

    }

    public Queue<Node> find_path_bfs(Node start, Node end, Grid grid) {

        Queue<Node> queue = new LinkedList<>();
        Queue<Node> path = new LinkedList<>();

        queue.add(start);
        grid.mark_visited(start.getR(), start.getC());

        while (!queue.isEmpty()) {
            Node node = queue.remove();
            path.add(node);

            if (node == end) break;

            ArrayList<Node> adj_nodes = get_neighbours(node, grid);
            for (Node n : adj_nodes) {
                if (!(n.getVal() == "V")) {
                    queue.add(n);
                    n.setVal("V");
                }
            }
        }
        return path;
    }

I took a look at this post , that suggests saving the path to the current node as well, but I'm not sure how to implement this in Java as I would have to have a queue that stores both a Node and a List of Nodes together as one item. My Java is a little rusty, so I'm not sure if this is possible or if there is a better way.

Answer 1

Your issue is that right now you're using the Queue<Node> path variable as a visited list, rather than a path list. Instead of building the path as you go, store references to each child node's parent and then traverse back through this. Something like this:

public ArrayList<Node> find_path_bfs(Node start, Node end, Grid grid) {

    Queue<Node> queue = new LinkedList<>();
    List<Node> path = new ArrayList<>();

    queue.add(start);
    grid.mark_visited(start.getR(), start.getC());

    Map<Node, Node> parentMap = new HashMap<>();

    Node node = start;
    parentMap.put(start, null); // start node has no parent, so we end path reconstruction here
    while (!queue.isEmpty()) {
        node = queue.remove();
        // path.add(node);

        if (node == end) break;

        ArrayList<Node> adj_nodes = get_neighbours(node, grid);
        for (Node n : adj_nodes) {
            if (!(n.getVal() == "V")) {
                queue.add(n);
                parentMap.put(n, node); // mark current node as parent to neighbor
                n.setVal("V");
            }
        }
    }

    Node curr = node;
    while (curr != null) {
        path.add(0, curr);
        curr = parentMap.get(curr);
    }

    return path;
}

I changed path to an ArrayList so that you can insert at the beginning (since you're reconstructing the path from the last node, rather than the first one). Alternatively, you could keep it as a Queue and reverse the order of the elements.