Dijkstra's algorithm - priority queue issue

问题

I have a Graph class with a bunch of nodes, edges, etc. and I'm trying to perform Dijkstra's algorithm. I start off adding all the nodes to a priority queue. Each node has a boolean flag for whether it is already 'known' or not, a reference to the node that comes before it, and an int dist field that stores its length from the source node. After adding all the nodes to the PQ and then flagging the source node appropriately, I've noticed that the wrong node is pulled off the PQ first. It should be that the node with the smallest dist field comes off first (since they are all initialized to a a very high number except for the source, the first node off the PQ should be the source... except it isn't for some reason). Below is my code for the algorithm followed by my compare method within my Node class.

    public void dijkstra() throws IOException {
    buildGraph_u();
    PriorityQueue<Node> pq = new PriorityQueue<>(200, new Node());

    for (int y = 0; y < input.size(); y++) {
        Node v = input.get(array.get(y));
        v.dist = 99999;
        v.known = false;
        v.prnode = null;
        pq.add(v);
    }

    source.dist = 0;
    source.known = true;
    source.prnode = null;
    int c=1;
    while(c != input.size()) {
        Node v = pq.remove();
        //System.out.println(v.name);
                    //^ Prints a node that isn't the source
        v.known = true;
        c++;
        List<Edge> listOfEdges = getAdjacent(v);
        for (int x = 0; x < listOfEdges.size(); x++) {
            Edge edge = listOfEdges.get(x);
            Node w = edge.to;
            if (!w.known) {
                int cvw = edge.weight;
                if (v.dist + cvw < w.dist) {
                    w.dist = v.dist + cvw;
                    w.prnode = v;
                }
            }
        }
    }
}

public int compare (Node d1, Node d2) {
       int dist1 = d1.dist;
       int dist2 = d2.dist;
       if (dist1 > dist2) 
         return 1;
       else if (dist1 < dist2) 
         return -1;
       else 
         return 0;
     }

Can anyone help me find the issue with my PQ?

回答1:

The priority queue uses assumption that order doesn't change after you will insert the element.

So instead of inserting all of the elements to priority queue you can:

Start with just one node.
Loop while priority queue is not empty.
Do nothing, if element is "known".
Whenever you find smaller distance add it to priority queue with "right" weight.
So you need to store a sth else in priority queue, a pair: distance at insertion time, node itself.

来源：https://stackoverflow.com/questions/13594699/dijkstras-algorithm-priority-queue-issue

标签

java

priority-queue

dijkstra