[英]What's wrong with this implementation of Tarjan's algorithm?
我已经在Wikipedia的文章之后实现了Tarjan的算法,但是遇到了问题。 我想做的是找到所有大小大于1的强连接组件。使用较小的输入,一切正常,但是,当使用input.txt进行测试时,该算法开始表现出奇怪的行为-更改n
(例如1,-10、3)程序给出不同的结果。 这就是我无法理解的。 该算法不应依赖于初始值,因为n
仅用作时间戳。 如果有什么想法请分享,这对我有很大帮助,谢谢。
这是代码:
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Stack;
public class GraphAlgorithms {
HashMap<String, HashSet<String>> graph;
int n;
public GraphAlgorithms(String fileName) {
graph = new HashMap<String, HashSet<String>>();
try (BufferedReader reader = new BufferedReader(new FileReader(fileName))) {
String line;
while ((line = reader.readLine()) != null) {
String[] tokens = line.split(" +");
HashSet<String> to = new HashSet<String>();
for (int i = 1; i < tokens.length; i++)
to.add(tokens[i]);
graph.put(tokens[0], to);
}
} catch (IOException e) {
e.printStackTrace();
}
}
public GraphAlgorithms(HashMap<String, HashSet<String>> graph) {
this.graph = graph;
}
private void strongConnect(String v, HashMap<String, Integer> index,
HashMap<String, Integer> lowlink, Stack<String> stack,
HashSet<String> inStack, ArrayList<HashSet<String>> ans)
{
index.put(v, n);
lowlink.put(v, n);
++n;
stack.push(v);
inStack.add(v);
HashSet<String> successors = graph.get(v);
for(String w : successors) {
if (!index.containsKey(w)) {
strongConnect(w, index, lowlink, stack, inStack, ans);
if (lowlink.get(w) < lowlink.get(v))
lowlink.put(v, lowlink.get(w));
}
else if (inStack.contains(w)) {
if (index.get(w) < lowlink.get(v))
lowlink.put(v, index.get(w));
}
}
if (index.get(v) == lowlink.get(v)) {
HashSet<String> connectedComponent = new HashSet<String>();
String w;
do {
w = stack.pop();
inStack.remove(w);
connectedComponent.add(w);
}
while (!w.equals(v));
ans.add(connectedComponent);
}
}
public ArrayList<HashSet<String>> getStronglyConnectedComponents() {
HashMap<String, Integer> index = new HashMap<String, Integer>();
HashMap<String, Integer> lowlink = new HashMap<String, Integer>();
Stack <String> stack = new Stack<String>();
HashSet<String> inStack = new HashSet<String>();
ArrayList<HashSet<String>> ans = new ArrayList<HashSet<String>>();
n = 0;
for (String v : graph.keySet()) {
if (!index.containsKey(v))
strongConnect(v, index, lowlink, stack, inStack, ans);
}
return ans;
}
public void printComponentsLargerThanOne() {
for (HashSet<String> component : getStronglyConnectedComponents()) {
if (component.size() > 1) {
for (String element : component) {
System.out.print(element + " ");
}
System.out.println();
}
}
}
}
和主班
public class Main {
public static void main(String[] args) {
GraphAlgorithms ga = new GraphAlgorithms("input.txt");
ga.printComponentsLargerThanOne();
}
}
由于Java显然并不总是实习类型为java.lang.Integer
对象,因此该行
if (index.get(v) == lowlink.get(v)) {
应该
if (index.get(v).equals(lowlink.get(v))) {
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