70 lines
2.0 KiB
Java
70 lines
2.0 KiB
Java
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import java.util.*;
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import java.util.stream.Stream;
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// https://en.wikipedia.org/wiki/Kosaraju%27s_algorithm
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public class SCCKosaraju {
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public static List<List<Integer>> scc(List<Integer>[] graph) {
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int n = graph.length;
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boolean[] used = new boolean[n];
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List<Integer> order = new ArrayList<>();
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for (int i = 0; i < n; i++)
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if (!used[i])
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dfs(graph, used, order, i);
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List<Integer>[] reverseGraph = Stream.generate(ArrayList::new).limit(n).toArray(List[]::new);
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for (int i = 0; i < n; i++)
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for (int j : graph[i])
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reverseGraph[j].add(i);
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List<List<Integer>> components = new ArrayList<>();
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Arrays.fill(used, false);
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Collections.reverse(order);
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for (int u : order)
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if (!used[u]) {
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List<Integer> component = new ArrayList<>();
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dfs(reverseGraph, used, component, u);
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components.add(component);
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}
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return components;
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}
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static void dfs(List<Integer>[] graph, boolean[] used, List<Integer> res, int u) {
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used[u] = true;
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for (int v : graph[u])
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if (!used[v])
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dfs(graph, used, res, v);
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res.add(u);
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}
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// DAG of strongly connected components
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public static List<Integer>[] sccGraph(List<Integer>[] graph, List<List<Integer>> components) {
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int[] comp = new int[graph.length];
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for (int i = 0; i < components.size(); i++)
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for (int u : components.get(i))
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comp[u] = i;
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List<Integer>[] g = Stream.generate(ArrayList::new).limit(components.size()).toArray(List[]::new);
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Set<Long> edges = new HashSet<>();
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for (int u = 0; u < graph.length; u++)
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for (int v : graph[u])
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if (comp[u] != comp[v] && edges.add(((long) comp[u] << 32) + comp[v]))
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g[comp[u]].add(comp[v]);
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return g;
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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<Integer>[] g = Stream.generate(ArrayList::new).limit(3).toArray(List[]::new);
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g[2].add(0);
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g[2].add(1);
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g[0].add(1);
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g[1].add(0);
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List<List<Integer>> components = scc(g);
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System.out.println(components);
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System.out.println(Arrays.toString(sccGraph(g, components)));
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}
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}
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