Question

Background

I am trying to code Dijkstra’s algorithm in O(mlogn) time, where m is the number of edges and n is the number of nodes. I am using to find the shortest path between a given starting node and a given ending node. And I’m pretty new at this.

Here is the algorithm I have come up with:

Assume the graph is represented by an adjacency matrix and each node has a row index.

Initialize starting node distance to zero, and all other nodes to inifinity, in the heap.

Create a list of shortest paths, equal to the number of nodes in the graph, set to 0.

While the index of the node that corresponds to the minimum element in the heap 
has no value in the list of shortest paths and heap has node distances, do:
    Remove the minimum node distance from the heap, and bubble as necessary to fill the removed node.
    Put the minimum node distance into the list of shortest paths at its row index.

    For all nodes that were adjacent to the node with the minimum distance (that was just removed), do:
      Update the distances in the heap for the current node, using the following calculation:
        min((deleted node distance + adjacent edge weight), current node's distance)
    Reorganize the heap to be a minimum heap.

Return value in the list of shortest paths at the location of the end node.

This is O(mlogn) because you only update the distances once per edge.

“It takes linear time
to initialize the heap, and then we perform m updates at a cost of O(log n) each for a total time of O(mlog n).” – http://www.cs.cmu.edu/~avrim/451f07/lectures/lect1011.pdf

Problem

In order to update the distances from the starting vertex in the correct location in the heap, insertions to the heap must be key-value pairs – with the key being the node (row index) and the value being the distance.

There are lecture slides online that say each entry in a priority queue ADT is a key-value pair (otherwise, how could it prioritize?).

Question

The methods for PriorityQueue have at most one parameter, so how do you insert a key associated with a value?

This must be done in a single file with a specific name (i.e. It is my understanding that I can’t make a KeyValuePair class implementing Comparator).

I’d love to hear your thoughts.

Answer 1

To use JDK’s implementation of priority queue for your application, you can maintain a Map<Key, Value> in addition to PriorityQueue<Value>. In your case, Key represents a node and Value is an object that holds the shortest distance to a node. To update the distance to a node, you first look up its corresponding distance object in the map. Then, you remove the distance object from the priority queue. Next, you update the distance object. Finally, you insert the distance object back in the priority queue.