寻找图中的有向环-java
作者:互联网
判断图中是否有一个有向环:
edgeTo 数组找到有向环的所有顶点
onStack 调用dfs时将其设置为true,调用结束时设为false。
public class DirectedCycle { private boolean[] marked; // marked[v] = has vertex v been marked? private int[] edgeTo; // edgeTo[v] = previous vertex on path to v private boolean[] onStack; // onStack[v] = is vertex on the stack? private Stack<Integer> cycle; // directed cycle (or null if no such cycle) /** * Determines whether the digraph {@code G} has a directed cycle and, if so, * finds such a cycle. * @param G the digraph */ public DirectedCycle(Digraph G) { marked = new boolean[G.V()]; onStack = new boolean[G.V()]; edgeTo = new int[G.V()]; for (int v = 0; v < G.V(); v++) if (!marked[v] && cycle == null) dfs(G, v); } // run DFS and find a directed cycle (if one exists) private void dfs(Digraph G, int v) { onStack[v] = true; marked[v] = true; for (int w : G.adj(v)) { // short circuit if directed cycle found if (cycle != null) return; // found new vertex, so recur else if (!marked[w]) { edgeTo[w] = v; dfs(G, w); } // trace back directed cycle else if (onStack[w]) { cycle = new Stack<Integer>(); for (int x = v; x != w; x = edgeTo[x]) { cycle.push(x); } cycle.push(w); cycle.push(v); assert check(); } } onStack[v] = false; } /** * Does the digraph have a directed cycle? * @return {@code true} if the digraph has a directed cycle, {@code false} otherwise */ public boolean hasCycle() { return cycle != null; } /** * Returns a directed cycle if the digraph has a directed cycle, and {@code null} otherwise. * @return a directed cycle (as an iterable) if the digraph has a directed cycle, * and {@code null} otherwise */ public Iterable<Integer> cycle() { return cycle; } // certify that digraph has a directed cycle if it reports one private boolean check() { if (hasCycle()) { // verify cycle int first = -1, last = -1; for (int v : cycle()) { if (first == -1) first = v; last = v; } if (first != last) { System.err.printf("cycle begins with %d and ends with %d\n", first, last); return false; } } return true; } /** * Unit tests the {@code DirectedCycle} 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); DirectedCycle finder = new DirectedCycle(G); if (finder.hasCycle()) { StdOut.print("Directed cycle: "); for (int v : finder.cycle()) { StdOut.print(v + " "); } StdOut.println(); } else { StdOut.println("No directed cycle"); } StdOut.println(); } }
有向环为3-4-5-3
标签:directed,java,int,寻找,marked,return,图中,new,cycle 来源: https://www.cnblogs.com/hequnwang/p/14307630.html