Below is the syntax highlighted version of NonrecursiveDirectedCycle.java
from §4.2 Directed Graphs.

/****************************************************************************** * Compilation: javac NonrecursiveDirectedCycle.java * Execution: java NonrecursiveDirectedCycle input.txt * Dependencies: Digraph.java Stack.java StdOut.java In.java * Data files: https://algs4.cs.princeton.edu/42digraph/tinyDG.txt * https://algs4.cs.princeton.edu/42digraph/tinyDAG.txt * * Finds a directed cycle in a digraph using nonrecursive DFS. * Runs in O(E + V) time. * * % java NonrecursiveDirectedCycle tinyDG.txt * Directed cycle: 3 5 4 3 * * % java NonrecursiveDirectedCycle tinyDAG.txt * No directed cycle * ******************************************************************************/ import java.util.Iterator; /** * The {@code NonrecursiveDirectedCycle} class represents a data type for * determining whether a digraph has a directed cycle. * The <em>hasCycle</em> operation determines whether the digraph has * a directed cycle and, and of so, the <em>cycle</em> operation * returns one. * <p> * This implementation uses a nonrecursive depth-first search. * The constructor takes time proportional to <em>V</em> + <em>E</em> * (in the worst case), where <em>V</em> is the number of vertices * and <em>E</em> is the number of edges. * Each instance method takes Θ(1) time. * It uses Θ(<em>V</em>) extra space (not including the digraph). * <p> * See {@link NonrecursiveTopological} to compute a topological order if the * digraph is acyclic. * <p> * For additional documentation, * see <a href="https://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 NonrecursiveDirectedCycle { 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 NonrecursiveDirectedCycle(Digraph G) { int[] edgeTo = new int[G.V()]; // edgeTo[v] = previous vertex on path to v boolean[] marked = new boolean[G.V()]; // marked[v] = has vertex v been marked? boolean[] onStack = new boolean[G.V()]; // onStack[v] = is vertex on the stack? Stack<Integer> stack = new Stack<Integer>(); // 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(); for (int s = 0; s < G.V(); s++) { if (!marked[s]) { onStack[s] = true; marked[s] = true; stack.push(s); while (!stack.isEmpty()) { int v = stack.peek(); if (adj[v].hasNext()) { int w = adj[v].next(); if (!marked[w]) { // discovered vertex w for the first time marked[w] = true; edgeTo[w] = v; stack.push(w); onStack[w] = true; } // 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(); return; } } else { // v's adjacency list is exhausted int vCopy = stack.pop(); assert v == vCopy; 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 NonrecursiveDirectedCycle} 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); NonrecursiveDirectedCycle finder = new NonrecursiveDirectedCycle(G); if (finder.hasCycle()) { StdOut.print("Directed cycle: "); for (int v : finder.cycle()) { StdOut.print(v + " "); } StdOut.println(); } else { StdOut.println("No directed cycle"); } } }

Last updated: Thu Aug 11 09:26:00 EDT 2022.