图的拓扑排序

Posted 2020-12-10 laydown

import java.util.*;

/**
 * 图的拓扑排序
 * 1)从图中找到所有入度为0的点输出
 * 2)把图中入度为0的点删掉，继续寻找入度为0的点输出，周而复始
 * 3)图的所有节点被删除后，依次输出的顺序就是拓扑排序
 * 要求：有向图且无环
 * 应用：事件安排、编译顺序
 */
public class TopoSort {

    // 图有向且无环
    public static List<Node> sortedTopology(Graph graph) {
        // key：某一个node
        // value：剩余的入度
        HashMap<Node, Integer> inMap = new HashMap<>();
        // 剩余入度为0的点，才能进这个队列
        Queue<Node> zeroInQueue = new LinkedList<>();

        for (Node node : graph.nodes.values()) {
            inMap.put(node, node.in);
            if (node.in == 0) {
                zeroInQueue.add(node);
            }
        }
        // 拓扑排序的结果，依次加入result
        List<Node> result = new ArrayList<>();
        while (!zeroInQueue.isEmpty()) {
            Node cur = zeroInQueue.poll();
            result.add(cur);
            for (Node next : cur.nexts) {
                inMap.put(next, inMap.get(next) - 1);
                if (inMap.get(next) == 0) {
                    zeroInQueue.add(next);
                }
            }
        }
        return result;
    }

    class Graph {

        public HashMap<Integer, Node> nodes;

        public HashSet<Edge> edges;

        public Graph() {
            nodes = new HashMap<>();
            edges = new HashSet<>();
        }

    }

    class Node {

        public int value;

        public int in;

        public int out;

        public ArrayList<Node> nexts;

        public ArrayList<Edge> edges;

        public Node(int value) {
            this.value = value;
            nexts = new ArrayList<>();
            edges = new ArrayList<>();
        }

    }

    class Edge {

        // 权重
        public int weight;

        public Node from;

        public Node to;

        public Edge(int weight, Node from, Node to) {
            this.weight = weight;
            this.from = from;
            this.to = to;
        }

    }

}

/* 如有意见或建议，欢迎评论区留言；如发现代码有误，欢迎批评指正 */