cs146/10-labMST/Assignment.java

package labMST;

import java.io.*;
import java.util.*;

public class Assignment {

    static class Graph {
        private int n; // No. of vertices
        private List<List<Integer>> adj; // An array of adjacency lists
        private Map<String, Integer> weightMap;

        public Graph(int n) {
            this.n = n;
            this.adj = new ArrayList<>(n);
            for (int i = 0; i < n; i++) {
                this.adj.add(new LinkedList<>());
            }
            this.weightMap = new HashMap<>();
        }

        public void addEdge(int u, int v) {
            this.adj.get(u).add(v);
        }

        public void addEdgeWeight(int u, int v, int w) {
            String e = u + "," + v;
            this.weightMap.put(e, w);
        }

        public int getWeight(int u, int v) {
            String e = u + "," + v;
            return this.weightMap.get(e);
        }

        public List<Integer> getAdjacent(int u) {
            return this.adj.get(u);
        }

        public int getLength() {
            return this.n;
        }

        public void show() {
            for (int v = 0; v < n; v++) {
                System.out.print(v + ":");
                for (int i : this.adj.get(v)) {
                    System.out.print(i + ";");
                }
                System.out.println();
            }
        }
    }

    static class QNode {
        int id;
        int key;
        boolean isValid;
        int pi;

        public QNode() {
            id = -1;
            key = -1;
            isValid = true;
            pi = -1;
        }
    }

    // Implement Prim's Algorithm for finding the Minimum Spanning Tree.
    // Return the QNode for each vertex in the graph, indicating the parent (pi)
    // of every vertex in the resulting tree. Other fields of QNode are optional
    // but may be useful in your implementation.
    public static List<QNode> Prim(Graph g) {
        // Initialize the QNode list
        List<QNode> qNodes = new ArrayList<>(g.getLength());
        for (int i = 0; i < g.getLength(); i++) {
            qNodes.add(new QNode());
            qNodes.get(i).id = i;
            // Set key to max value for lowest priority
            qNodes.get(i).key = Integer.MAX_VALUE;
        }

        // Start from the first vertex
        qNodes.get(0).key = 0;

        // Priority queue to select the vertex with the minimum key as the next to be added to the MST
        PriorityQueue<QNode> pq = new PriorityQueue<>(Comparator.comparingInt(qNode -> qNode.key));
        // Add the first vertex to the priority queue
        pq.add(qNodes.get(0));

        while (!pq.isEmpty()) {
            // Get the vertex with the minimum key
            QNode u = pq.poll();
            // Mark it as part of the MST
            u.isValid = false;

            // For each adjacent vertex,
            // if it is not already in the MST and the weight is less than the current key
            // update the key and parent (pi)
            for (int v : g.getAdjacent(u.id)) {
                if (qNodes.get(v).isValid && g.getWeight(u.id, v) < qNodes.get(v).key) {
                    qNodes.get(v).key = g.getWeight(u.id, v);
                    qNodes.get(v).pi = u.id;
                    pq.add(qNodes.get(v));
                }
            }
        }

        return qNodes;
    }

    public static void run(String inputPath) {
        try (BufferedReader br = new BufferedReader(new FileReader(inputPath))) {

            // Read graph
            // n is # of nodes. nodes are indexed by 0, 1, 2, ..., n -1
            int n = Integer.parseInt(br.readLine().trim());

            // # of edges.
            int m = Integer.parseInt(br.readLine().trim());

            // Build the graph based on the edges on remaining input lines
            Graph g = new Graph(n);
            for (int i = 0; i < m; i++) {
                String[] parts = br.readLine().trim().split(" ");
                int s = Integer.parseInt(parts[0]);
                int t = Integer.parseInt(parts[1]);
                g.addEdge(s, t);
                int w = Integer.parseInt(parts[2]);
                g.addEdgeWeight(s, t, w);
                g.addEdge(t, s);
                g.addEdgeWeight(t, s, w);
            }

            // Your implementation must return a list of QNode. One QNode for
            // each vertex, in the order 0, 1, 2, ..., n-1
            List<QNode> pQNode = Prim(g);

            // The correctness of your implementation will be judged by printing
            // the parent of each vertex, line by line
            for (int i = 1; i < n; i++) {
                System.out.println(pQNode.get(i).pi);
            }

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}