(Leetcode) 323 - Number of Connected Components in an Undirected Graph

public class Solution {
    public int countComponents(int n, int[][] edges) {
        Map<Integer, Vertex> vertexMap = new HashMap<>();
        int lastGroupId = 0;

        for (int[] edge : edges) {
            Vertex v1 = vertexMap.getOrDefault(edge[0], new Vertex(edge[0]));
            Vertex v2 = vertexMap.getOrDefault(edge[1], new Vertex(edge[1]));

            v1.edges.add(v2);
            v2.edges.add(v1);

            vertexMap.put(edge[0], v1);
            vertexMap.put(edge[1], v2);
        }

        for (int i = 0; i < n; i++) {
            Vertex vertex = vertexMap.get(i);
            if (vertex == null) {
                lastGroupId++;
            } else {
                // 0 이 아닐 경우는 이미 탐색한 케이스므로 스킵
                if (vertex.groupId == 0) {
                    lastGroupId++;
                    dfs(vertex, lastGroupId);
                }
            }
        }

        return lastGroupId;
    }

    public void dfs(Vertex vertex, int groupId) {
        vertex.groupId = groupId;
        for (Vertex connected : vertex.edges) {
            if (connected.groupId == 0) {
                dfs(connected, groupId);
            }
        }
    }
}

class Vertex {
    int id;
    int groupId;
    List<Vertex> edges;

    public Vertex(int id) {
        this.id = id;
        this.edges = new ArrayList<>();
    }
}