edu.princeton.cs.algs4.BreadthFirstDirectedPaths Maven / Gradle / Ivy
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/******************************************************************************
* Compilation: javac BreadthFirstDirectedPaths.java
* Execution: java BreadthFirstDirectedPaths digraph.txt s
* Dependencies: Digraph.java Queue.java Stack.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
*
* Run breadth-first search on a digraph.
* Runs in O(E + V) time.
*
* % java BreadthFirstDirectedPaths tinyDG.txt 3
* 3 to 0 (2): 3->2->0
* 3 to 1 (3): 3->2->0->1
* 3 to 2 (1): 3->2
* 3 to 3 (0): 3
* 3 to 4 (2): 3->5->4
* 3 to 5 (1): 3->5
* 3 to 6 (-): not connected
* 3 to 7 (-): not connected
* 3 to 8 (-): not connected
* 3 to 9 (-): not connected
* 3 to 10 (-): not connected
* 3 to 11 (-): not connected
* 3 to 12 (-): not connected
*
******************************************************************************/
package edu.princeton.cs.algs4;
/**
* The {@code BreadthDirectedFirstPaths} class represents a data type for finding
* shortest paths (number of edges) from a source vertex s
* (or set of source vertices) to every other vertex in the digraph.
*
* This implementation uses breadth-first search.
* The constructor takes time proportional to V + E,
* where V is the number of vertices and E is the number of edges.
* Each call to {@link #distTo(int)} and {@link #hasPathTo(int)} takes constant time;
* each call to {@link #pathTo(int)} takes time proportional to the length
* of the path.
* It uses extra space (not including the digraph) proportional to V.
*
* For additional documentation,
* see Section 4.2 of
* Algorithms, 4th Edition by Robert Sedgewick and Kevin Wayne.
*
* @author Robert Sedgewick
* @author Kevin Wayne
*/
public class BreadthFirstDirectedPaths {
private static final int INFINITY = Integer.MAX_VALUE;
private boolean[] marked; // marked[v] = is there an s->v path?
private int[] edgeTo; // edgeTo[v] = last edge on shortest s->v path
private int[] distTo; // distTo[v] = length of shortest s->v path
/**
* Computes the shortest path from {@code s} and every other vertex in graph {@code G}.
* @param G the digraph
* @param s the source vertex
* @throws IllegalArgumentException unless {@code 0 <= v < V}
*/
public BreadthFirstDirectedPaths(Digraph G, int s) {
marked = new boolean[G.V()];
distTo = new int[G.V()];
edgeTo = new int[G.V()];
for (int v = 0; v < G.V(); v++)
distTo[v] = INFINITY;
validateVertex(s);
bfs(G, s);
}
/**
* Computes the shortest path from any one of the source vertices in {@code sources}
* to every other vertex in graph {@code G}.
* @param G the digraph
* @param sources the source vertices
* @throws IllegalArgumentException unless each vertex {@code v} in
* {@code sources} satisfies {@code 0 <= v < V}
*/
public BreadthFirstDirectedPaths(Digraph G, Iterable sources) {
marked = new boolean[G.V()];
distTo = new int[G.V()];
edgeTo = new int[G.V()];
for (int v = 0; v < G.V(); v++)
distTo[v] = INFINITY;
validateVertices(sources);
bfs(G, sources);
}
// BFS from single source
private void bfs(Digraph G, int s) {
Queue q = new Queue();
marked[s] = true;
distTo[s] = 0;
q.enqueue(s);
while (!q.isEmpty()) {
int v = q.dequeue();
for (int w : G.adj(v)) {
if (!marked[w]) {
edgeTo[w] = v;
distTo[w] = distTo[v] + 1;
marked[w] = true;
q.enqueue(w);
}
}
}
}
// BFS from multiple sources
private void bfs(Digraph G, Iterable sources) {
Queue q = new Queue();
for (int s : sources) {
marked[s] = true;
distTo[s] = 0;
q.enqueue(s);
}
while (!q.isEmpty()) {
int v = q.dequeue();
for (int w : G.adj(v)) {
if (!marked[w]) {
edgeTo[w] = v;
distTo[w] = distTo[v] + 1;
marked[w] = true;
q.enqueue(w);
}
}
}
}
/**
* Is there a directed path from the source {@code s} (or sources) to vertex {@code v}?
* @param v the vertex
* @return {@code true} if there is a directed path, {@code false} otherwise
* @throws IllegalArgumentException unless {@code 0 <= v < V}
*/
public boolean hasPathTo(int v) {
validateVertex(v);
return marked[v];
}
/**
* Returns the number of edges in a shortest path from the source {@code s}
* (or sources) to vertex {@code v}?
* @param v the vertex
* @return the number of edges in a shortest path
* @throws IllegalArgumentException unless {@code 0 <= v < V}
*/
public int distTo(int v) {
validateVertex(v);
return distTo[v];
}
/**
* Returns a shortest path from {@code s} (or sources) to {@code v}, or
* {@code null} if no such path.
* @param v the vertex
* @return the sequence of vertices on a shortest path, as an Iterable
* @throws IllegalArgumentException unless {@code 0 <= v < V}
*/
public Iterable pathTo(int v) {
validateVertex(v);
if (!hasPathTo(v)) return null;
Stack path = new Stack();
int x;
for (x = v; distTo[x] != 0; x = edgeTo[x])
path.push(x);
path.push(x);
return path;
}
// 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));
}
// throw an IllegalArgumentException unless {@code 0 <= v < V}
private void validateVertices(Iterable vertices) {
if (vertices == null) {
throw new IllegalArgumentException("argument is null");
}
int V = marked.length;
for (int v : vertices) {
if (v < 0 || v >= V) {
throw new IllegalArgumentException("vertex " + v + " is not between 0 and " + (V-1));
}
}
}
/**
* Unit tests the {@code BreadthFirstDirectedPaths} 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);
int s = Integer.parseInt(args[1]);
BreadthFirstDirectedPaths bfs = new BreadthFirstDirectedPaths(G, s);
for (int v = 0; v < G.V(); v++) {
if (bfs.hasPathTo(v)) {
StdOut.printf("%d to %d (%d): ", s, v, bfs.distTo(v));
for (int x : bfs.pathTo(v)) {
if (x == s) StdOut.print(x);
else StdOut.print("->" + x);
}
StdOut.println();
}
else {
StdOut.printf("%d to %d (-): not connected\n", s, v);
}
}
}
}
/******************************************************************************
* Copyright 2002-2018, 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.
******************************************************************************/