英文:
Tarjan's Algorithm implementation, Low value node issue
问题
我正在尝试在图的一组节点上执行Tarjan算法，我可以成功找到强连通分量（SCC），但是根节点或低值节点始终不正确，即使对于只有一个元素的SCC，根节点也不匹配该元素。
例如：
0-3 是低值节点
[82-97]
529-529 是低值节点
[69-81]
379-379 是低值节点
[57-68]
1619-1619 是低值节点
[136-137]
415-415 是低值节点
[45-56]
顶部是低值节点，底部是组成SCC的单个元素
我目前的代码如下：
package Structs;
import java.util.*;
public class TarjanSCC {
private int id;
private boolean[] onStack;
private int[] ids, low;
private Deque<Integer> stack;
private Graph<BasicBlock> graph;
private Map<BasicBlock, Integer> nodeMap;
private BasicBlock[] nodes;
private static final int UNVISITED = -1;
public TarjanSCC(Graph<BasicBlock> graph) {
this.graph = graph;
}
public void runTSCC() {
runTSCC(graph);
}
public void runTSCC(Graph<BasicBlock> nodeGraph) {
// 初始化排序所需的值
this.nodes = nodeGraph.getNodes().toArray(new BasicBlock[nodeGraph.size()]);
int size = nodes.length;
this.ids = new int[size];
this.low = new int[size];
this.onStack = new boolean[size];
this.stack = new ArrayDeque<>();
this.nodeMap = new HashMap<>();
// 将所有节点标记为未使用，并将节点放入映射中，以便可以轻松地从节点检索索引
for (int i = 0; i < size; i++) {
nodeMap.put(nodes[i], i);
ids[i] = UNVISITED;
}
// 对每个未访问的节点调用DFS算法
for (int i = 0; i < size; i++) {
if (ids[i] == UNVISITED) dfs(i, nodeGraph);
}
}
private void dfs(int at, Graph<BasicBlock> nodeGraph) {
ids[at] = low[at] = id++;
stack.push(at);
onStack[at] = true;
// 访问所有图的邻居，并标记为已访问并添加到堆栈中
for (BasicBlock edge : nodeGraph.getEdges(nodes[at])) {
int nodeArrayIndex = nodeMap.get(edge);
if (ids[nodeArrayIndex] == UNVISITED) {
dfs(nodeArrayIndex, nodeGraph);
low[at] = Math.min(low[at], low[nodeArrayIndex]);
} else if (onStack[nodeArrayIndex]) low[at] = Math.min(low[at], nodeArrayIndex);
}
// 我们已经访问了所有邻居，现在开始将堆栈清空，直到达到SCC的起点
if (low[at] == ids[at]) {
List<BasicBlock> sccList = new ArrayList<>();
for (int node = stack.pop(); ; node = stack.pop()) {
onStack[node] = false;
sccList.add(0, nodes[node]);
if (node == at) {
System.out.println(nodes[low[at]] + " 是低值节点");
System.out.println(sccList);
break;
}
}
}
}
}
我怀疑问题在于设置低值节点。不需要其他类信息，因为从图中检索边没有问题。
英文:
I am trying to execute tarjans algorithm on a set of nodes from a graph, I can succesfully find Strongly Connected Components, however the root or the low value node is always off, even for SCC's with only 1 element the root node does not match that element
For Example:
0-3 IS LOW VALUE NODE
[82-97]
529-529 IS LOW VALUE NODE
[69-81]
379-379 IS LOW VALUE NODE
[57-68]
1619-1619 IS LOW VALUE NODE
[136-137]
415-415 IS LOW VALUE NODE
[45-56]
where the top is the low value node and the bottom is the 1 element that makes up the SCC
The code I currently have is here
package Structs;
import java.util.*;
public class TarjanSCC {
private int id;
private boolean[] onStack;
private int[] ids, low;
private Deque<Integer> stack;
private Graph<BasicBlock> graph;
private Map<BasicBlock, Integer> nodeMap;
private BasicBlock[] nodes;
private static final int UNVISITED = -1;
public TarjanSCC(Graph<BasicBlock> graph) {
this.graph = graph;
}
public void runTSCC() {
runTSCC(graph);
}
public void runTSCC(Graph<BasicBlock> nodeGraph) {
//Initialize values for sorting
this.nodes = nodeGraph.getNodes().toArray(new BasicBlock[nodeGraph.size()]);
int size = nodes.length;
this.ids = new int[size];
this.low = new int[size];
this.onStack = new boolean[size];
this.stack = new ArrayDeque<>();
this.nodeMap = new HashMap<>();
//Mark all nodes as unused, place nodes in a map so index is easily retrievable from node
for(int i = 0; i < size; i++) {
nodeMap.put(nodes[i], i);
ids[i] = UNVISITED;
}
//Invoke the DFS algorithm on each unvisited node
for(int i = 0; i < size; i++) {
if (ids[i] == UNVISITED) dfs(i, nodeGraph);
}
}
private void dfs(int at, Graph<BasicBlock> nodeGraph) {
ids[at] = low[at] = id++;
stack.push(at);
onStack[at] = true;
//Visit All Neighbours of graph and mark as visited and add to stack,
for (BasicBlock edge : nodeGraph.getEdges(nodes[at])) {
int nodeArrayIndex = nodeMap.get(edge);
if (ids[nodeArrayIndex] == UNVISITED) {
dfs(nodeArrayIndex, nodeGraph);
low[at] = Math.min(low[at], low[nodeArrayIndex]);
}
else if (onStack[nodeArrayIndex]) low[at] = Math.min(low[at], nodeArrayIndex);
}
//We've visited all the neighours, lets start emptying the stack until we're at the start of the SCC
if (low[at] == ids[at]) {
List<BasicBlock> sccList = new ArrayList<>();
for (int node = stack.pop();; node = stack.pop()) {
onStack[node] = false;
sccList.add(0, nodes[node]);
if (node == at) {
System.out.println(nodes[low[at]] + " IS LOW VALUE NODE");
System.out.println(sccList);
break;
}
}
}
}
}
I suspect the issue lies with setting the low values.
I do not believe any other class info is required as there is no issues with retrieving edges from the graph
答案1
得分: 0
我错误地索引了根节点，nodes[at] 是访问根节点的正确方式。
英文:
I was indexing the root node wrong nodes[at] is the proper way to access the root node
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