[백준/자바] 11724 - 연결 요소의 개수

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.StringTokenizer;

public class Main {

    static ArrayList<Integer>[] al;
    static boolean[] visited;

    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer st = new StringTokenizer(br.readLine());

        int N = Integer.parseInt(st.nextToken());
        int M = Integer.parseInt(st.nextToken());

        al = new ArrayList[N + 1];
        visited = new boolean[N + 1];

        for (int i = 1; i <= N; i++)
            al[i] = new ArrayList<>();

        for (int i = 0; i < M; i++) {
            st = new StringTokenizer(br.readLine());
            int start = Integer.parseInt(st.nextToken());
            int end = Integer.parseInt(st.nextToken());

            al[start].add(end);
            al[end].add(start);
        }

        int count = 0;

        for (int i = 1; i <= N; i++) {
            if (!visited[i]) {
                count++;
                dfs(i);
            }
        }

        System.out.println(count);
    }

    private static void dfs(int k) {
        if (visited[k]) return;

        visited[k] = true;
        for (int i : al[k]) {
            if (!visited[i]) dfs(i);
        }
    }
}