Below is the syntax highlighted version of Digraph.java
from §4.2 Directed Graphs.
/****************************************************************************** * Compilation: javac Digraph.java * Execution: java Digraph filename.txt * Dependencies: Bag.java In.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 * * A graph, implemented using an array of lists. * Parallel edges and self-loops are permitted. * * % java Digraph tinyDG.txt * 13 vertices, 22 edges * 0: 5 1 * 1: * 2: 0 3 * 3: 5 2 * 4: 3 2 * 5: 4 * 6: 9 4 8 0 * 7: 6 9 * 8: 6 * 9: 11 10 * 10: 12 * 11: 4 12 * 12: 9 * ******************************************************************************/ import java.util.NoSuchElementException; /** * The {@code Digraph} class represents a directed graph of vertices * named 0 through <em>V</em> - 1. * It supports the following two primary operations: add an edge to the digraph, * iterate over all of the vertices adjacent from a given vertex. * It also provides * methods for returning the indegree or outdegree of a vertex, * the number of vertices <em>V</em> in the digraph, * the number of edges <em>E</em> in the digraph, and the reverse digraph. * Parallel edges and self-loops are permitted. * <p> * This implementation uses an <em>adjacency-lists representation</em>, which * is a vertex-indexed array of {@link Bag} objects. * It uses Θ(<em>E</em> + <em>V</em>) space, where <em>E</em> is * the number of edges and <em>V</em> is the number of vertices. * The <code>reverse()</code> method takes Θ(<em>E</em> + <em>V</em>) time * and space; all other instance methods take Θ(1) time. (Though, iterating over * the vertices returned by {@link #adj(int)} takes time proportional * to the outdegree of the vertex.) * Constructing an empty digraph with <em>V</em> vertices takes * Θ(<em>V</em>) time; constructing a digraph with <em>E</em> edges * and <em>V</em> vertices takes Θ(<em>E</em> + <em>V</em>) time. * <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 Digraph { private static final String NEWLINE = System.getProperty("line.separator"); private final int V; // number of vertices in this digraph private int E; // number of edges in this digraph private Bag<Integer>[] adj; // adj[v] = adjacency list for vertex v private int[] indegree; // indegree[v] = indegree of vertex v /** * Initializes an empty digraph with <em>V</em> vertices. * * @param V the number of vertices * @throws IllegalArgumentException if {@code V < 0} */ public Digraph(int V) { if (V < 0) throw new IllegalArgumentException("Number of vertices in a Digraph must be non-negative"); this.V = V; this.E = 0; indegree = new int[V]; adj = (Bag<Integer>[]) new Bag[V]; for (int v = 0; v < V; v++) { adj[v] = new Bag<Integer>(); } } /** * Initializes a digraph from the specified input stream. * The format is the number of vertices <em>V</em>, * followed by the number of edges <em>E</em>, * followed by <em>E</em> pairs of vertices, with each entry separated by whitespace. * * @param in the input stream * @throws IllegalArgumentException if {@code in} is {@code null} * @throws IllegalArgumentException if the endpoints of any edge are not in prescribed range * @throws IllegalArgumentException if the number of vertices or edges is negative * @throws IllegalArgumentException if the input stream is in the wrong format */ public Digraph(In in) { if (in == null) throw new IllegalArgumentException("argument is null"); try { this.V = in.readInt(); if (V < 0) throw new IllegalArgumentException("number of vertices in a Digraph must be non-negative"); indegree = new int[V]; adj = (Bag<Integer>[]) new Bag[V]; for (int v = 0; v < V; v++) { adj[v] = new Bag<Integer>(); } int E = in.readInt(); if (E < 0) throw new IllegalArgumentException("number of edges in a Digraph must be non-negative"); for (int i = 0; i < E; i++) { int v = in.readInt(); int w = in.readInt(); addEdge(v, w); } } catch (NoSuchElementException e) { throw new IllegalArgumentException("invalid input format in Digraph constructor", e); } } /** * Initializes a new digraph that is a deep copy of the specified digraph. * * @param G the digraph to copy * @throws IllegalArgumentException if {@code G} is {@code null} */ public Digraph(Digraph G) { if (G == null) throw new IllegalArgumentException("argument is null"); this.V = G.V(); this.E = G.E(); if (V < 0) throw new IllegalArgumentException("Number of vertices in a Digraph must be non-negative"); // update indegrees indegree = new int[V]; for (int v = 0; v < V; v++) this.indegree[v] = G.indegree(v); // update adjacency lists adj = (Bag<Integer>[]) new Bag[V]; for (int v = 0; v < V; v++) { adj[v] = new Bag<Integer>(); } for (int v = 0; v < G.V(); v++) { // reverse so that adjacency list is in same order as original Stack<Integer> reverse = new Stack<Integer>(); for (int w : G.adj[v]) { reverse.push(w); } for (int w : reverse) { adj[v].add(w); } } } /** * Returns the number of vertices in this digraph. * * @return the number of vertices in this digraph */ public int V() { return V; } /** * Returns the number of edges in this digraph. * * @return the number of edges in this digraph */ public int E() { return E; } // throw an IllegalArgumentException unless {@code 0 <= v < V} private void validateVertex(int v) { if (v < 0 || v >= V) throw new IllegalArgumentException("vertex " + v + " is not between 0 and " + (V-1)); } /** * Adds the directed edge v→w to this digraph. * * @param v the tail vertex * @param w the head vertex * @throws IllegalArgumentException unless both {@code 0 <= v < V} and {@code 0 <= w < V} */ public void addEdge(int v, int w) { validateVertex(v); validateVertex(w); adj[v].add(w); indegree[w]++; E++; } /** * Returns the vertices adjacent from vertex {@code v} in this digraph. * * @param v the vertex * @return the vertices adjacent from vertex {@code v} in this digraph, as an iterable * @throws IllegalArgumentException unless {@code 0 <= v < V} */ public Iterable<Integer> adj(int v) { validateVertex(v); return adj[v]; } /** * Returns the number of directed edges incident from vertex {@code v}. * This is known as the <em>outdegree</em> of vertex {@code v}. * * @param v the vertex * @return the outdegree of vertex {@code v} * @throws IllegalArgumentException unless {@code 0 <= v < V} */ public int outdegree(int v) { validateVertex(v); return adj[v].size(); } /** * Returns the number of directed edges incident to vertex {@code v}. * This is known as the <em>indegree</em> of vertex {@code v}. * * @param v the vertex * @return the indegree of vertex {@code v} * @throws IllegalArgumentException unless {@code 0 <= v < V} */ public int indegree(int v) { validateVertex(v); return indegree[v]; } /** * Returns the reverse of the digraph. * * @return the reverse of the digraph */ public Digraph reverse() { Digraph reverse = new Digraph(V); for (int v = 0; v < V; v++) { for (int w : adj(v)) { reverse.addEdge(w, v); } } return reverse; } /** * Returns a string representation of the graph. * * @return the number of vertices <em>V</em>, followed by the number of edges <em>E</em>, * followed by the <em>V</em> adjacency lists */ public String toString() { StringBuilder s = new StringBuilder(); s.append(V + " vertices, " + E + " edges " + NEWLINE); for (int v = 0; v < V; v++) { s.append(String.format("%d: ", v)); for (int w : adj[v]) { s.append(String.format("%d ", w)); } s.append(NEWLINE); } return s.toString(); } /** * Unit tests the {@code Digraph} 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); StdOut.println(G); } }