Below is the syntax highlighted version of TarjanSCC.java.
/****************************************************************************** * Compilation: javac TarjanSCC.java * Execution: Java TarjanSCC V E * Dependencies: Digraph.java Stack.java TransitiveClosure.java StdOut.java * Data files: https://algs4.cs.princeton.edu/42digraph/tinyDG.txt * https://algs4.cs.princeton.edu/42digraph/mediumDG.txt * https://algs4.cs.princeton.edu/42digraph/largeDG.txt * * Compute the strongly-connected components of a digraph using * Tarjan's algorithm. * * Runs in O(E + V) time. * * % java TarjanSCC tinyDG.txt * 5 components * 1 * 0 2 3 4 5 * 9 10 11 12 * 6 8 * 7 * ******************************************************************************/ package edu.princeton.cs.algs4; /** * The {@code TarjanSCC} class represents a data type for * determining the strong components in a digraph. * The <em>id</em> operation determines in which strong component * a given vertex lies; the <em>areStronglyConnected</em> operation * determines whether two vertices are in the same strong component; * and the <em>count</em> operation determines the number of strong * components. * <p> * The <em>component identifier</em> of a component is one of the * vertices in the strong component: two vertices have the same component * identifier if and only if they are in the same strong component. * <p> * This implementation uses Tarjan's algorithm. * The constructor takes Θ(<em>V</em> + <em>E</em>) time, * where <em>V</em> is the number of vertices and <em>E</em> is the * number of edges. * Each instance method takes Θ(1) time. * It uses Θ(<em>V</em>) extra space (not including the digraph). * For alternative implementations of the same API, see * {@link KosarajuSharirSCC} and {@link GabowSCC}. * <p> * For additional documentation, * see <a href="https://algs4.cs.princeton.edu/42digraph">Section 4.2</a> of * <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne. * * @author Robert Sedgewick * @author Kevin Wayne */ public class TarjanSCC { private boolean[] marked; // marked[v] = has v been visited? private int[] id; // id[v] = id of strong component containing v private int[] low; // low[v] = low number of v private int pre; // preorder number counter private int count; // number of strongly-connected components private Stack<Integer> stack; /** * Computes the strong components of the digraph {@code G}. * @param G the digraph */ public TarjanSCC(Digraph G) { marked = new boolean[G.V()]; stack = new Stack<Integer>(); id = new int[G.V()]; low = new int[G.V()]; for (int v = 0; v < G.V(); v++) { if (!marked[v]) dfs(G, v); } // check that id[] gives strong components assert check(G); } private void dfs(Digraph G, int v) { marked[v] = true; low[v] = pre++; int min = low[v]; stack.push(v); for (int w : G.adj(v)) { if (!marked[w]) dfs(G, w); if (low[w] < min) min = low[w]; } if (min < low[v]) { low[v] = min; return; } int w; do { w = stack.pop(); id[w] = count; low[w] = G.V(); } while (w != v); count++; } /** * Returns the number of strong components. * @return the number of strong components */ public int count() { return count; } /** * Are vertices {@code v} and {@code w} in the same strong component? * @param v one vertex * @param w the other vertex * @return {@code true} if vertices {@code v} and {@code w} are in the same * strong component, and {@code false} otherwise * @throws IllegalArgumentException unless {@code 0 <= v < V} * @throws IllegalArgumentException unless {@code 0 <= w < V} */ public boolean stronglyConnected(int v, int w) { validateVertex(v); validateVertex(w); return id[v] == id[w]; } /** * Returns the component id of the strong component containing vertex {@code v}. * @param v the vertex * @return the component id of the strong component containing vertex {@code v} * @throws IllegalArgumentException unless {@code 0 <= v < V} */ public int id(int v) { validateVertex(v); return id[v]; } // does the id[] array contain the strongly connected components? private boolean check(Digraph G) { TransitiveClosure tc = new TransitiveClosure(G); for (int v = 0; v < G.V(); v++) { for (int w = 0; w < G.V(); w++) { if (stronglyConnected(v, w) != (tc.reachable(v, w) && tc.reachable(w, v))) return false; } } return true; } // throw an IllegalArgumentException unless {@code 0 <= v < V} private void validateVertex(int v) { int V = marked.length; if (v < 0 || v >= V) throw new IllegalArgumentException("vertex " + v + " is not between 0 and " + (V-1)); } /** * Unit tests the {@code TarjanSCC} data type. * * @param args the command-line arguments */ public static void main(String[] args) { In in = new In(args[0]); Digraph G = new Digraph(in); TarjanSCC scc = new TarjanSCC(G); // number of connected components int m = scc.count(); StdOut.println(m + " components"); // compute list of vertices in each strong component Queue<Integer>[] components = (Queue<Integer>[]) new Queue[m]; for (int i = 0; i < m; i++) { components[i] = new Queue<Integer>(); } for (int v = 0; v < G.V(); v++) { components[scc.id(v)].enqueue(v); } // print results for (int i = 0; i < m; i++) { for (int v : components[i]) { StdOut.print(v + " "); } StdOut.println(); } } } /****************************************************************************** * Copyright 2002-2022, Robert Sedgewick and Kevin Wayne. * * This file is part of algs4.jar, which accompanies the textbook * * Algorithms, 4th edition by Robert Sedgewick and Kevin Wayne, * Addison-Wesley Professional, 2011, ISBN 0-321-57351-X. * http://algs4.cs.princeton.edu * * * algs4.jar is free software: you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation, either version 3 of the License, or * (at your option) any later version. * * algs4.jar is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with algs4.jar. If not, see http://www.gnu.org/licenses. ******************************************************************************/