最小生成树之Prim算法

CSDN

package com.zhang.reflection.面试.算法模版.图.最小生成树;
import java.util.*;
/**
 * 最小生成树，从点的角度考虑，从那个点出发都行
 */
public class Prim {
    //图
    static class Graph{
        //点集<编号，实际的点>
        public HashMap<Integer, Node> nodes;
        //边集
        public HashSet<Edge> edges;
        public Graph(){
            nodes=new HashMap<>();
            edges=new HashSet<>();
        }
    }
    //点
    static class Node{
        //点上的值
        public int value;
        //点的入度
        public int in;
        //点的出度
        public int out;
        //相邻节点集,以该节点为起点
        public ArrayList<Node> nexts;
        //相邻边集，以该节点为起点
        public ArrayList<Edge> edges;
        public Node(int value){
            this.value=value;
            in=0;
            out=0;
            nexts=new ArrayList<>();
            edges=new ArrayList<>();
        }
    }
    //边
    static class Edge{
        //边得权值
        public int weight;
        //边的起点
        public Node from;
        //边的终点
        public Node to;
        public Edge(int weight, Node from, Node to){
            this.weight=weight;
            this.from=from;
            this.to=to;
        }
    }
    //比较器
    public static class EdgeCompator implements Comparator<Edge> {
        @Override
        public int compare(Edge o1, Edge o2) {
            return o1.weight - o2.weight;
        }
    }
    //普里姆算法
    public static Set<Edge> primMST(Graph graph) {
        HashSet<Node> set = new HashSet<>();//标记作用
        PriorityQueue<Edge> priorityQueue = new PriorityQueue<Edge>(new EdgeCompator());
        HashSet<Edge> res = new HashSet<>();
        //针对森林结构
        for (Node node : graph.nodes.values()) {
            if (!set.contains(node)) {
                set.add(node);
                priorityQueue.addAll(node.edges);
                while (!priorityQueue.isEmpty()) {
                    Edge poll = priorityQueue.poll();
                    Node toNode = poll.to;
                    if (!set.contains(toNode)) {
                        res.add(poll);
                        set.add(toNode);
                        priorityQueue.addAll(toNode.edges);
                    }
                }
            }
        }
        return res;
    }
}