Graph.java

/******************************************************************************
 *  Compilation:  javac Graph.java
 *  Execution:    java Graph input.txt
 *  Dependencies: Bag.java In.java StdOut.java
 *  Data files:   http://algs4.cs.princeton.edu/41graph/tinyG.txt
 *
 *  A graph, implemented using an array of sets.
 *  Parallel edges and self-loops allowed.
 *
 *  % java Graph tinyG.txt
 *  13 vertices, 13 edges
 *  0: 6 2 1 5
 *  1: 0
 *  2: 0
 *  3: 5 4
 *  4: 5 6 3
 *  5: 3 4 0
 *  6: 0 4
 *  7: 8
 *  8: 7
 *  9: 11 10 12
 *  10: 9
 *  11: 9 12
 *  12: 11 9
 *
 *  % java Graph mediumG.txt
 *  250 vertices, 1273 edges
 *  0: 225 222 211 209 204 202 191 176 163 160 149 114 97 80 68 59 58 49 44 24 15
 *  1: 220 203 200 194 189 164 150 130 107 72
 *  2: 141 110 108 86 79 51 42 18 14
 *  ...
 *
 ******************************************************************************/


/**
 *  The <tt>Graph</tt> class represents an undirected graph of vertices
 *  named 0 through <em>V</em> - 1.
 *  It supports the following two primary operations: add an edge to the graph,
 *  iterate over all of the vertices adjacent to a vertex. It also provides
 *  methods for returning the number of vertices <em>V</em> and the number
 *  of edges <em>E</em>. Parallel edges and self-loops are permitted.
 *  <p>
 *  This implementation uses an adjacency-lists representation, which
 *  is a vertex-indexed array of {@link Bag} objects.
 *  All operations take constant time (in the worst case) except
 *  iterating over the vertices adjacent to a given vertex, which takes
 *  time proportional to the number of such vertices.
 *  <p>
 *  For additional documentation, see <a href="http://algs4.cs.princeton.edu/41graph">Section 4.1</a>
 *  of <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
 *
 *  @author Robert Sedgewick
 *  @author Kevin Wayne
 */
public class Graph {
    private static final String NEWLINE = System.getProperty("line.separator");

    private final int V;
    private int E;
    private Bag<Integer>[] adj;

    /**
     * Initializes an empty graph with <tt>V</tt> vertices and 0 edges.
     * param V the number of vertices
     *
     * @param  V number of vertices
     * @throws IllegalArgumentException if <tt>V</tt> < 0
     */
    public Graph(int V) {
        if (V < 0) throw new IllegalArgumentException("Number of vertices must be nonnegative");
        this.V = V;
        this.E = 0;
        adj = (Bag<Integer>[]) new Bag[V];
        for (int v = 0; v < V; v++) {
            adj[v] = new Bag<Integer>();
        }
    }

    /**
     * Initializes a graph from an 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 IndexOutOfBoundsException if the endpoints of any edge are not in prescribed range
     * @throws IllegalArgumentException if the number of vertices or edges is negative
     */
    public Graph(In in) {
        this(in.readInt());
        int E = in.readInt();
        if (E < 0) throw new IllegalArgumentException("Number of edges must be nonnegative");
        for (int i = 0; i < E; i++) {
            int v = in.readInt();
            int w = in.readInt();
            addEdge(v, w);
        }
    }

    /**
     * Initializes a new graph that is a deep copy of <tt>G</tt>.
     *
     * @param  G the graph to copy
     */
    public Graph(Graph G) {
        this(G.V());
        this.E = G.E();
        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 graph.
     *
     * @return the number of vertices in this graph
     */
    public int V() {
        return V;
    }

    /**
     * Returns the number of edges in this graph.
     *
     * @return the number of edges in this graph
     */
    public int E() {
        return E;
    }

    // throw an IndexOutOfBoundsException unless 0 <= v < V
    private void validateVertex(int v) {
        if (v < 0 || v >= V)
            throw new IndexOutOfBoundsException("vertex " + v + " is not between 0 and " + (V-1));
    }

    /**
     * Adds the undirected edge v-w to this graph.
     *
     * @param  v one vertex in the edge
     * @param  w the other vertex in the edge
     * @throws IndexOutOfBoundsException unless both 0 <= v < V and 0 <= w < V
     */
    public void addEdge(int v, int w) {
        validateVertex(v);
        validateVertex(w);
        E++;
        adj[v].add(w);
        adj[w].add(v);
    }


    /**
     * Returns the vertices adjacent to vertex <tt>v</tt>.
     *
     * @param  v the vertex
     * @return the vertices adjacent to vertex <tt>v</tt>, as an iterable
     * @throws IndexOutOfBoundsException unless 0 <= v < V
     */
    public Iterable<Integer> adj(int v) {
        validateVertex(v);
        return adj[v];
    }


    /**
     * Returns a string representation of this 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(v + ": ");
            for (int w : adj[v]) {
                s.append(w + " ");
            }
            s.append(NEWLINE);
        }
        return s.toString();
    }


    /**
     * Unit tests the <tt>Graph</tt> data type.
     */
    public static void main(String[] args) {
        In in = new In(args[0]);
        Graph G = new Graph(in);
        StdOut.println(G);
    }

}