Cod sursa(job #2087401)

Utilizator rontavValentin Mocanu rontav Data 13 decembrie 2017 16:35:19
Problema Algoritmul lui Dijkstra Scor 50
Compilator java Status done
Runda Arhiva educationala Marime 3.74 kb
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.util.*;

public class Main {
    public static void main(String[] args) throws IOException {
        FileReader fileReader = new FileReader("dijkstra.in");
        BufferedReader bufferedReader = new BufferedReader(fileReader);

        Scanner scanner = new Scanner(bufferedReader);

        int nodes = scanner.nextInt();
        int noEdges = scanner.nextInt();

        Map<Integer, Graph.Vertex> graph = new HashMap<>(noEdges);

        for (int i = 0; i < noEdges; i++) {
            int v1 = scanner.nextInt();
            int v2 = scanner.nextInt();
            int dist = scanner.nextInt();

            if (!graph.containsKey(v1)) {
                graph.put(v1, new Graph.Vertex(v1));
            }
            if (!graph.containsKey(v2)) {
                graph.put(v2, new Graph.Vertex(v2));
            }

            graph.get(v1).neighbours.add(v2);
            graph.get(v1).neighbours.add(dist);
        }
        bufferedReader.close();

        Graph g = new Graph(graph);
        g.dijkstra(1);

        StringBuilder output = new StringBuilder();

        for (int i = 2; i <= nodes; i++) {
            Graph.Vertex vertex = g.graph.get(i);

            if (vertex == null || vertex.dist == Integer.MAX_VALUE) {
                output.append("0");
            } else {
                output.append(vertex.dist);
            }
            output.append(" ");
        }
        BufferedWriter writer = new BufferedWriter(new FileWriter("dijkstra.out"));
        writer.write(output.toString());
        writer.close();

    }
}

class Graph {
    public final Map<Integer, Vertex> graph;

    public static class Vertex implements Comparable<Vertex> {
        public final int id;
        public int dist = Integer.MAX_VALUE; // MAX_VALUE assumed to be infinity
        public Vertex previous = null;
        public ArrayList<Integer> neighbours = new ArrayList<>();

        public Vertex(int id) {
            this.id = id;
        }

        public int compareTo(Vertex other) {
            return Double.compare(dist, other.dist);
        }

        @Override
        public String toString() {
            return "(" + id + ", " + dist + ")";
        }
    }

    public Graph(Map<Integer, Vertex> graph) {
        this.graph = graph;
    }

    /**
     * Runs dijkstra using a specified source vertex
     */
    public void dijkstra(Integer startName) {
        if (!graph.containsKey(startName)) {
            System.err.printf("Graph doesn't contain start vertex \"%s\"\n", startName);
            return;
        }
        final Vertex source = graph.get(startName);
        source.dist = 0;
        source.previous = source;

        PriorityQueue<Vertex> heap = new PriorityQueue<Vertex>();

        heap.add(source);

        Vertex u, v;
        int dist;

        while (!heap.isEmpty()) {
            u = heap.remove(); // vertex with shortest distance (first iteration will return source)

            if (u.dist == Integer.MAX_VALUE) {
                break;
                // we can ignore u (and any other remaining vertices) since they are unreachable
            }

            //look at distances to each neighbour
            for (int i = 0; i < u.neighbours.size(); i += 2) {
                v = graph.get(u.neighbours.get(i));
                dist = u.neighbours.get(i + 1);

                int alternateDist = u.dist + dist;
                if (alternateDist < v.dist) {
                    // shorter path to neighbour found
                    heap.remove(v);
                    v.dist = alternateDist;
                    v.previous = u;
                    heap.add(v);
                }
            }
        }
    }
}