Difficulties Implementing Dijkstra's Algorithm in Java - Results Varying with Priority Queue Implementation
I'm currently working on implementing Dijkstra's algorithm to find the shortest path in a graph using Java, but I'm facing some inconsistencies in the results depending on the priority queue implementation I use. Here's the version of my algorithm using `PriorityQueue`: ```java import java.util.*; class Graph { private final Map<Integer, List<Edge>> adjacencyList; public Graph() { adjacencyList = new HashMap<>(); } public void addEdge(int source, int destination, int weight) { adjacencyList.putIfAbsent(source, new ArrayList<>()); adjacencyList.get(source).add(new Edge(destination, weight)); } public Map<Integer, Integer> dijkstra(int start) { Map<Integer, Integer> distances = new HashMap<>(); PriorityQueue<Edge> priorityQueue = new PriorityQueue<>(Comparator.comparingInt(edge -> edge.weight)); priorityQueue.add(new Edge(start, 0)); distances.put(start, 0); while (!priorityQueue.isEmpty()) { Edge current = priorityQueue.poll(); int currentNode = current.destination; int currentWeight = current.weight; for (Edge edge : adjacencyList.getOrDefault(currentNode, new ArrayList<>())) { int newDistance = currentWeight + edge.weight; if (newDistance < distances.getOrDefault(edge.destination, Integer.MAX_VALUE)) { distances.put(edge.destination, newDistance); priorityQueue.add(new Edge(edge.destination, newDistance)); } } } return distances; } } class Edge { int destination; int weight; public Edge(int destination, int weight) { this.destination = destination; this.weight = weight; } } ``` I've also tried using `ArrayList` to manage my priority queue manually, but the results are different. With the `PriorityQueue`, I sometimes get higher values for the shortest path than expected, especially in graphs with multiple edges between nodes. For example, in a simple graph: ```java Graph graph = new Graph(); graph.addEdge(1, 2, 1); graph.addEdge(1, 3, 4); graph.addEdge(2, 3, 2); graph.addEdge(2, 4, 5); graph.addEdge(3, 4, 1); Map<Integer, Integer> distances = graph.dijkstra(1); System.out.println(distances); ``` With this setup, I'm getting a distance of 7 for node 4, but I expected it to be 5 (1 -> 2 -> 3 -> 4). I've verified that the edges are being added correctly and even printed debug information for the distances map at each iteration. Am I missing something in my implementation? Should I approach the priority queue differently? Any insights would be greatly appreciated! This is part of a larger CLI tool I'm building. Is there a better approach?