NonrecursiveDirectedDFS.java
5.4 KB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
/******************************************************************************
* Compilation: javac NonrecursiveDirectedDFS.java
* Execution: java NonrecursiveDirectedDFS digraph.txt s
* Dependencies: Digraph.java Queue.java Stack.java StdOut.java
* Data files: http://algs4.cs.princeton.edu/42digraph/tinyDG.txt
* http://algs4.cs.princeton.edu/42digraph/mediumDG.txt
* http://algs4.cs.princeton.edu/42digraph/largeDG.txt
*
* Run nonrecurisve depth-first search on an directed graph.
* Runs in O(E + V) time.
*
* Explores the vertices in exactly the same order as DirectedDFS.java.
*
*
* % java NonrecursiveDirectedDFS tinyDG.txt 1
* 1
*
* % java NonrecursiveDirectedDFS tinyDG.txt 2
* 0 1 2 3 4 5
*
******************************************************************************/
package edu.princeton.cs.algs4;
import java.util.Iterator;
/**
* The {@code NonrecursiveDirectedDFS} class represents a data type for finding
* the vertices reachable from a source vertex <em>s</em> in the digraph.
* <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 digraph) proportional to <em>V</em>.
* <p>
* For additional documentation,
* see <a href="http://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 NonrecursiveDirectedDFS {
private boolean[] marked; // marked[v] = is there an s->v path?
/**
* Computes the vertices reachable from the source vertex {@code s} in the digraph {@code G}.
* @param G the digraph
* @param s the source vertex
* @throws IllegalArgumentException unless {@code 0 <= s < V}
*/
public NonrecursiveDirectedDFS(Digraph 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} reachable from the source vertex {@code s}?
* @param v the vertex
* @return {@code true} if vertex {@code v} is reachable from 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 NonrecursiveDirectedDFS} 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);
int s = Integer.parseInt(args[1]);
NonrecursiveDirectedDFS dfs = new NonrecursiveDirectedDFS(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.
******************************************************************************/