判斷圖是否是無環圖
阿新 • • 發佈:2021-01-20
判斷一個圖是否為無環圖:
完整程式碼
public class Cycle { private boolean[] marked; private int[] edgeTo; private Stack<Integer> cycle; /** * Determines whether the undirected graph {@code G} has a cycle and, * if so, finds such a cycle. * * @param G the undirected graph*/ public Cycle(Graph G) { if (hasSelfLoop(G)) return; if (hasParallelEdges(G)) return; marked = new boolean[G.V()]; edgeTo = new int[G.V()]; for (int v = 0; v < G.V(); v++) if (!marked[v]) dfs(G, -1, v); } // does this graph have a self loop?// side effect: initialize cycle to be self loop private boolean hasSelfLoop(Graph G) { for (int v = 0; v < G.V(); v++) { for (int w : G.adj(v)) { if (v == w) { cycle = new Stack<Integer>(); cycle.push(v); cycle.push(v);return true; } } } return false; } // does this graph have two parallel edges? // side effect: initialize cycle to be two parallel edges private boolean hasParallelEdges(Graph G) { marked = new boolean[G.V()]; for (int v = 0; v < G.V(); v++) { // check for parallel edges incident to v for (int w : G.adj(v)) { if (marked[w]) { cycle = new Stack<Integer>(); cycle.push(v); cycle.push(w); cycle.push(v); return true; } marked[w] = true; } // reset so marked[v] = false for all v for (int w : G.adj(v)) { marked[w] = false; } } return false; } /** * Returns true if the graph {@code G} has a cycle. * * @return {@code true} if the graph has a cycle; {@code false} otherwise */ public boolean hasCycle() { return cycle != null; } /** * Returns a cycle in the graph {@code G}. * @return a cycle if the graph {@code G} has a cycle, * and {@code null} otherwise */ public Iterable<Integer> cycle() { return cycle; } private void dfs(Graph G, int u, int v) { marked[v] = true; for (int w : G.adj(v)) { // short circuit if cycle already found if (cycle != null) return; if (!marked[w]) { edgeTo[w] = v; dfs(G, v, w); } // check for cycle (but disregard reverse of edge leading to v) else if (w != u) { cycle = new Stack<Integer>(); for (int x = v; x != w; x = edgeTo[x]) { cycle.push(x); } cycle.push(w); cycle.push(v); } } } /** * Unit tests the {@code Cycle} data type. * * @param args the command-line arguments */ public static void main(String[] args) { In in = new In(args[0]); Graph G = new Graph(in); Cycle finder = new Cycle(G); if (finder.hasCycle()) { for (int v : finder.cycle()) { StdOut.print(v + " "); } StdOut.println(); } else { StdOut.println("Graph is acyclic"); } } }