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87 lines
2.0 KiB
Java
87 lines
2.0 KiB
Java
import java.util.*;
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public class BellmanFord2 {
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static final int INF = Integer.MAX_VALUE / 2;
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public static class Edge {
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int u, v;
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int cost;
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public Edge(int u, int v, int cost) {
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this.u = u;
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this.v = v;
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this.cost = cost;
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}
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}
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public static boolean bellmanFord(int n, List<Edge> edges, int s, int[] dist, int[] pred) {
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Arrays.fill(pred, -1);
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Arrays.fill(dist, INF);
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dist[s] = 0;
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boolean updated = false;
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for (int step = 0; step < n; step++) {
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updated = false;
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for (Edge e : edges) {
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if (dist[e.u] == INF) continue;
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if (dist[e.v] > dist[e.u] + e.cost) {
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dist[e.v] = dist[e.u] + e.cost;
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dist[e.v] = Math.max(dist[e.v], -INF);
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pred[e.v] = e.u;
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updated = true;
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}
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}
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if (!updated)
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break;
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}
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// if updated is true then a negative cycle exists
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return updated == false;
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}
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public static int[] findNegativeCycle(int n, List<Edge> edges) {
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int[] pred = new int[n];
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Arrays.fill(pred, -1);
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int[] dist = new int[n];
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int last = -1;
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for (int step = 0; step < n; step++) {
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last = -1;
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for (Edge e : edges) {
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if (dist[e.u] == INF) continue;
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if (dist[e.v] > dist[e.u] + e.cost) {
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dist[e.v] = dist[e.u] + e.cost;
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dist[e.v] = Math.max(dist[e.v], -INF);
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pred[e.v] = e.u;
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last = e.v;
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}
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}
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if (last == -1)
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return null;
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}
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for (int i = 0; i < n; i++) {
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last = pred[last];
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}
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int[] p = new int[n];
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int cnt = 0;
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for (int u = last; u != last || cnt == 0; u = pred[u]) {
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p[cnt++] = u;
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}
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int[] cycle = new int[cnt];
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for (int i = 0; i < cycle.length; i++) {
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cycle[i] = p[--cnt];
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}
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return cycle;
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}
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// Usage example
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public static void main(String[] args) {
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List<Edge> edges = new ArrayList<>();
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edges.add(new Edge(0, 1, 1));
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edges.add(new Edge(1, 0, 1));
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edges.add(new Edge(1, 2, 1));
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edges.add(new Edge(2, 3, -10));
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edges.add(new Edge(3, 1, 1));
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int[] cycle = findNegativeCycle(4, edges);
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System.out.println(Arrays.toString(cycle));
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}
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}
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