拓扑排序BFS

拓扑排序BFS

import java.util.*;

public class TopologicalSort {
    public static List<Integer> kahnTopologicalSort(int n, List<List<Integer>> edges) {
        // 初始化入度数组和邻接表
        int[] inDegree = new int[n];
        List<List<Integer>> adjList = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            adjList.add(new ArrayList<>());
        }

        // 构建图
        for (List<Integer> edge : edges) {
            int u = edge.get(0);
            int v = edge.get(1);
            adjList.get(u).add(v);
            inDegree[v]++;
        }

        // 初始化队列，加入所有入度为0的节点
        Queue<Integer> queue = new LinkedList<>();
        for (int i = 0; i < n; i++) {
            if (inDegree[i] == 0) {
                queue.offer(i);
            }
        }

        List<Integer> result = new ArrayList<>();
        while (!queue.isEmpty()) {
            int u = queue.poll();
            result.add(u);

            // 遍历邻接节点，减少入度
            for (int v : adjList.get(u)) {
                inDegree[v]--;
                if (inDegree[v] == 0) {
                    queue.offer(v);
                }
            }
        }

        // 如果结果中节点数不等于图中节点数，说明存在环
        if (result.size() != n) {
            throw new RuntimeException("Graph contains a cycle, topological sort is not possible.");
        }

        return result;
    }

    public static void main(String[] args) {
        int n = 6; // 节点数
        List<List<Integer>> edges = Arrays.asList(
                Arrays.asList(0, 1),
                Arrays.asList(1, 2),
                Arrays.asList(3, 1),
                Arrays.asList(4, 0),
                Arrays.asList(4, 5)
        );

        List<Integer> sortedOrder = kahnTopologicalSort(n, edges);
        System.out.println("Topological Sort Order: " + sortedOrder);
    }
}