NonrecursiveDFS.java
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/******************************************************************************
* Compilation: javac NonrecursiveDFS.java
* Execution: java NonrecursiveDFS graph.txt s
* Dependencies: Graph.java Queue.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
*
* Run nonrecurisve depth-first search on an undirected graph.
* Runs in O(E + V) time.
*
* Explores the vertices in exactly the same order as DepthFirstSearch.java.
*
* % 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 NonrecursiveDFS tinyCG.txt 0
* 0 to 0 (0): 0
* 0 to 1 (1): 0-1
* 0 to 2 (1): 0-2
* 0 to 3 (2): 0-2-3
* 0 to 4 (2): 0-2-4
* 0 to 5 (1): 0-5
*
******************************************************************************/
package edu.princeton.cs.algs4;
import java.util.Iterator;
/**
* The {@code NonrecursiveDFS} class represents a data type for finding
* the vertices connected to a source vertex <em>s</em> in the undirected
* graph.
* <p>
* This implementation uses a nonrecursive version of depth-first search
* with an explicit stack.
* 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 NonrecursiveDFS {
private boolean[] marked; // marked[v] = is there an s-v path?
/**
* Computes the vertices connected to the source vertex {@code s} in the graph {@code G}.
* @param G the graph
* @param s the source vertex
* @throws IllegalArgumentException unless {@code 0 <= s < V}
*/
public NonrecursiveDFS(Graph G, int s) {
marked = new boolean[G.V()];
validateVertex(s);
// to be able to iterate over each adjacency list, keeping track of which
// vertex in each adjacency list needs to be explored next
Iterator<Integer>[] adj = (Iterator<Integer>[]) new Iterator[G.V()];
for (int v = 0; v < G.V(); v++)
adj[v] = G.adj(v).iterator();
// depth-first search using an explicit stack
Stack<Integer> stack = new Stack<Integer>();
marked[s] = true;
stack.push(s);
while (!stack.isEmpty()) {
int v = stack.peek();
if (adj[v].hasNext()) {
int w = adj[v].next();
// StdOut.printf("check %d\n", w);
if (!marked[w]) {
// discovered vertex w for the first time
marked[w] = true;
// edgeTo[w] = v;
stack.push(w);
// StdOut.printf("dfs(%d)\n", w);
}
}
else {
// StdOut.printf("%d done\n", v);
stack.pop();
}
}
}
/**
* Is vertex {@code v} connected to the source vertex {@code s}?
* @param v the vertex
* @return {@code true} if vertex {@code v} is connected to the source vertex {@code s},
* and {@code false} otherwise
* @throws IllegalArgumentException unless {@code 0 <= v < V}
*/
public boolean marked(int v) {
validateVertex(v);
return marked[v];
}
// 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 NonrecursiveDFS} 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]);
NonrecursiveDFS dfs = new NonrecursiveDFS(G, s);
for (int v = 0; v < G.V(); v++)
if (dfs.marked(v))
StdOut.print(v + " ");
StdOut.println();
}
}
/******************************************************************************
* 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.
******************************************************************************/