30 | 图的表示：如何存储微博、微信等社交网络中的好友关系

package com.controller.bootweb.demo.dsa;
import java.util.ArrayList;import java.util.List;import java.util.Map;import java.util.concurrent.ConcurrentHashMap;
public class WechatGraph {    private final Map<Integer, List<Integer>> adjacencyList;
    public WechatGraph() {        this.adjacencyList = new ConcurrentHashMap<>();    }
    public void addEdge(int v1, int v2) {        // Add edge v1 -> v2        adjacencyList.computeIfAbsent(v1, k -> new ArrayList<>()).add(v2);        // Add edge v2 -> v1 (for undirected graph)        adjacencyList.computeIfAbsent(v2, k -> new ArrayList<>()).add(v1);    }
    public List<Integer> getNeighbors(int vertex) {        return adjacencyList.getOrDefault(vertex, new ArrayList<>());    }
    public static void main(String[] args) {        WechatGraph graph = new WechatGraph();        graph.addEdge(1, 2);        graph.addEdge(2, 3);        graph.addEdge(3, 1);
        // Example: Get neighbors of vertex 1        List<Integer> neighbors = graph.getNeighbors(1);        System.out.println("Neighbors of vertex 1: " + neighbors);    }}

package com.controller.bootweb.demo.dsa;
import java.util.ArrayList;import java.util.Arrays;import java.util.List;import java.util.Map;import java.util.concurrent.ConcurrentHashMap;
public class WechatGraph2 {    private final int[][] adjacencyMatrix;    private final int vertices;
    public WechatGraph2(int vertices) {        this.vertices = vertices;        this.adjacencyMatrix = new int[vertices][vertices];    }
    public void addEdge(int v1, int v2) {        // Add edge v1 -> v2        adjacencyMatrix[v1][v2] = 1;        // Add edge v2 -> v1 (for undirected graph)        adjacencyMatrix[v2][v1] = 1;    }
    public int[][] getAdjacencyMatrix() {        return adjacencyMatrix;    }
    public static void main(String[] args) {        int numberOfVertices = 4;        WechatGraph2 graph = new WechatGraph2(numberOfVertices);        graph.addEdge(0, 1);        graph.addEdge(1, 2);        graph.addEdge(2, 0);
        // Example: Get adjacency matrix        int[][] matrix = graph.getAdjacencyMatrix();        System.out.println("Adjacency Matrix:");        for (int[] row : matrix) {            System.out.println(Arrays.toString(row));        }    }}