package com.zy.acs.manager.common.utils;

import java.util.*;

/**
 * Breadth-First Search
 * 1.可用于计算最短路径 或者 快速反推二叉树的根节点
 * https://www.bilibili.com/video/BV1Ks411575U/?vd_source=6f04e61f1b57e41e7986c65388d892fb
 * 2.如果连接有权重，那么把Queue变成PriorityQueue => Dijkstra
 * https://www.bilibili.com/video/BV1ts41157Sy/?spm_id_from=333.999.0.0&vd_source=6f04e61f1b57e41e7986c65388d892fb
 */
public class BFS {

    private final Map<String, List<String>> graph;

    private final Set<String> visited = new HashSet<>();

    // 可以通过BFS扫描节点连接关系，来获取节点的最近路径的父节点hash表
    private final Map<String, String> parent = new HashMap<>();

    public BFS(Map<String, List<String>> graph) {
        this.graph = graph;
    }

    public List<String> execute(String startNode) {
        if (null == graph) {
            throw new RuntimeException("the graph was empty");
        }

        List<String> list = new ArrayList<>();

        Queue<String> queue = new LinkedList<>();

        queue.add(startNode);
        visited.add(startNode);
        parent.put(startNode, null);

        while (queue.size() > 0) {
            String node = queue.poll();

            List<String> nears = graph.get(node);
            for (String near : nears) {
                if (!visited.contains(near)) {
                    queue.offer(near);
                    visited.add(near);

                    parent.put(near, node);
                }
            }

            list.add(node);
        }

        return list;
    }

    public static void main(String[] args) {
        Map<String, List<String>> graph = new HashMap<>();
        graph.put("A", new ArrayList<String>(){{add("B");add("C");}});
        graph.put("B", new ArrayList<String>(){{add("A");add("C");add("D");}});
        graph.put("C", new ArrayList<String>(){{add("A");add("B");add("D");add("E");}});
        graph.put("D", new ArrayList<String>(){{add("B");add("C");add("E");add("F");}});
        graph.put("E", new ArrayList<String>(){{add("C");add("D");}});
        graph.put("F", new ArrayList<String>(){{add("D");}});

        BFS bfs = new BFS(graph);
        bfs.execute("E");
        String endNode = "B";
        List<String> shortestPath = new ArrayList<>();
        while (endNode != null) {
            shortestPath.add(endNode);
            endNode = bfs.parent.get(endNode);
        }
        Collections.reverse(shortestPath);

        System.out.println("shortestPath: " + shortestPath);
    }

}