백준 11724번(Java)

import java.io.*;
import java.util.*;

public class Main {
		static List<ArrayList<Integer>> nearList = new ArrayList<>();
		static boolean node[];
		static int N, count;

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

			st = new StringTokenizer(br.readLine());
			N = Integer.parseInt(st.nextToken()); // 정점의 개수
			int M = Integer.parseInt(st.nextToken()); // 간선의 개수

			// node와 인접리스트 생성
			node = new boolean[N+1];
			for(int i=0; i<=N; i++) {
				nearList.add(new ArrayList<>());
			}

			while(M-- > 0) {
	 			st = new StringTokenizer(br.readLine());
				int x = Integer.parseInt(st.nextToken());
				int y = Integer.parseInt(st.nextToken());

				nearList.get(x).add(y);
				nearList.get(y).add(x);
			}

			int count = 0;
			for(int i=1; i<=N; i++) {

				if(node[i] == false) {
					BFS(i);
					count ++;
				}

			}

			System.out.println(count);

		} 

		static void BFS(int value) {
			Queue<Integer> queue = new LinkedList<>();
			queue.offer(value);

			while( !queue.isEmpty() ) {
				int num = queue.poll();

				int size = nearList.get(num).size();

				for(int i=0; i<size; i++) {
					int new_value = nearList.get(num).get(i);

					if(node[new_value] == false) {
						queue.offer(new_value);
						node[new_value] = true;
					}
				}

			}
		} 
}