DepthFirstPaths.java
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/******************************************************************************
* Compilation: javac DepthFirstPaths.java
* Execution: java DepthFirstPaths G s
* Dependencies: Graph.java Stack.java StdOut.java
* Data files: http://algs4.cs.princeton.edu/41graph/tinyCG.txt
* http://algs4.cs.princeton.edu/41graph/tinyG.txt
* http://algs4.cs.princeton.edu/41graph/mediumG.txt
* http://algs4.cs.princeton.edu/41graph/largeG.txt
*
* Run depth first search on an undirected graph.
* Runs in O(E + V) time.
*
* % java Graph tinyCG.txt
* 6 8
* 0: 2 1 5
* 1: 0 2
* 2: 0 1 3 4
* 3: 5 4 2
* 4: 3 2
* 5: 3 0
*
* % java DepthFirstPaths tinyCG.txt 0
* 0 to 0: 0
* 0 to 1: 0-2-1
* 0 to 2: 0-2
* 0 to 3: 0-2-3
* 0 to 4: 0-2-3-4
* 0 to 5: 0-2-3-5
*
******************************************************************************/
package edu.princeton.cs.algs4;
/**
* The {@code DepthFirstPaths} class represents a data type for finding
* paths from a source vertex <em>s</em> to every other vertex
* in an undirected graph.
* <p>
* This implementation uses depth-first search.
* The constructor takes time proportional to <em>V</em> + <em>E</em>,
* where <em>V</em> is the number of vertices and <em>E</em> is the number of edges.
* It uses extra space (not including the graph) proportional to <em>V</em>.
* <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 DepthFirstPaths {
private boolean[] marked; // marked[v] = is there an s-v path?
private int[] edgeTo; // edgeTo[v] = last edge on s-v path
private final int s; // source vertex
/**
* Computes a path between {@code s} and every other vertex in graph {@code G}.
* @param G the graph
* @param s the source vertex
* @throws IllegalArgumentException unless {@code 0 <= s < V}
*/
public DepthFirstPaths(Graph G, int s) {
this.s = s;
edgeTo = new int[G.V()];
marked = new boolean[G.V()];
validateVertex(s);
dfs(G, s);
}
// depth first search from v
private void dfs(Graph G, int v) {
marked[v] = true;
for (int w : G.adj(v)) {
if (!marked[w]) {
edgeTo[w] = v;
dfs(G, w);
}
}
}
/**
* Is there a path between the source vertex {@code s} and vertex {@code v}?
* @param v the vertex
* @return {@code true} if there is a path, {@code false} otherwise
* @throws IllegalArgumentException unless {@code 0 <= v < V}
*/
public boolean hasPathTo(int v) {
validateVertex(v);
return marked[v];
}
/**
* Returns a path between the source vertex {@code s} and vertex {@code v}, or
* {@code null} if no such path.
* @param v the vertex
* @return the sequence of vertices on a path between the source vertex
* {@code s} and vertex {@code v}, as an Iterable
* @throws IllegalArgumentException unless {@code 0 <= v < V}
*/
public Iterable<Integer> pathTo(int v) {
validateVertex(v);
if (!hasPathTo(v)) return null;
Stack<Integer> path = new Stack<Integer>();
for (int x = v; x != s; x = edgeTo[x])
path.push(x);
path.push(s);
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));
}
/**
* Unit tests the {@code DepthFirstPaths} data type.
*
* @param args the command-line arguments
*/
public static void main(String[] args) {
In in = new In(args[0]);
Graph G = new Graph(in);
int s = Integer.parseInt(args[1]);
DepthFirstPaths dfs = new DepthFirstPaths(G, s);
for (int v = 0; v < G.V(); v++) {
if (dfs.hasPathTo(v)) {
StdOut.printf("%d to %d: ", s, v);
for (int x : dfs.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 2002-2016, 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.
******************************************************************************/