92 lines
2.2 KiB
Java
92 lines
2.2 KiB
Java
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import java.util.*;
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import java.util.stream.Stream;
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public class BellmanFord {
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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 v, cost;
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public Edge(int v, int cost) {
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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(List<Edge>[] graph, 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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int n = graph.length;
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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 (int u = 0; u < n; u++) {
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if (dist[u] == INF) continue;
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for (Edge e : graph[u]) {
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if (dist[e.v] > dist[u] + e.cost) {
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dist[e.v] = dist[u] + e.cost;
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dist[e.v] = Math.max(dist[e.v], -INF);
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pred[e.v] = u;
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updated = true;
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}
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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(List<Edge>[] graph) {
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int n = graph.length;
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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 (int u = 0; u < n; u++) {
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if (dist[u] == INF) continue;
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for (Edge e : graph[u]) {
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if (dist[e.v] > dist[u] + e.cost) {
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dist[e.v] = Math.max(dist[u] + e.cost, -INF);
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dist[e.v] = Math.max(dist[e.v], -INF);
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pred[e.v] = u;
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last = e.v;
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}
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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>[] graph = Stream.generate(ArrayList::new).limit(4).toArray(List[]::new);
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graph[0].add(new Edge(1, 1));
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graph[1].add(new Edge(0, 1));
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graph[1].add(new Edge(2, 1));
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graph[2].add(new Edge(3, -10));
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graph[3].add(new Edge(1, 1));
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int[] cycle = findNegativeCycle(graph);
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System.out.println(Arrays.toString(cycle));
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}
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}
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