拓扑排序DFS

拓扑排序DFS

package nkbasc;

import java.util.*;

public class AB13 {
    private static List<Integer> topologicalSort(List<List<Integer>> graph, int numVertices) {
        List<Integer> result = new ArrayList<>();
        boolean[] visited = new boolean[numVertices];
        boolean[] recursionStack = new boolean[numVertices]; // 用于检测环
        boolean hasCycle = false;

        for (int i = 0; i < numVertices; i++) {
            if (!visited[i]) {
                if (dfs(graph, i, visited, recursionStack, result)) {
                    hasCycle = true;
                    break;
                }
            }
        }

        if (hasCycle) {
            System.out.println(-1);
            return Collections.emptyList();
        }

        Collections.reverse(result); // 拓扑排序的结果需要反转
        return result;
    }

    private static boolean dfs(List<List<Integer>> graph, int vertex, boolean[] visited, boolean[] recursionStack, List<Integer> result) {
        visited[vertex] = true;
        recursionStack[vertex] = true;

        for (int neighbor : graph.get(vertex)) {
            if (!visited[neighbor]) {
                if (dfs(graph, neighbor, visited, recursionStack, result)) {
                    return true;
                }
            } else if (recursionStack[neighbor]) {
                // 如果在递归栈中再次遇到该节点，说明存在环
                return true;
            }
        }

        recursionStack[vertex] = false;
        result.add(vertex);
        return false;
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int m = in.nextInt();

        List<List<Integer>> graph = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            graph.add(new ArrayList<>());
        }

        for (int i = 0; i < m; i++) {
            int u = in.nextInt() - 1;
            int v = in.nextInt() - 1;
            graph.get(u).add(v);
        }

        List<Integer> sortedOrder = topologicalSort(graph, n);
        if (!sortedOrder.isEmpty()) {
            for (int i = 0; i < sortedOrder.size(); i++) {
                System.out.print(sortedOrder.get(i) + 1);
                if (i < sortedOrder.size() - 1) {
                    System.out.print(" ");
                }
            }
        }
    }
}