[BOJ] 2606. 바이러스

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.Queue;
import java.util.Scanner;

public class Main {
	static int N, M;
	static ArrayList<Integer>[] edge;
	static boolean[] check;
	static int ans;
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		N = sc.nextInt();
		M = sc.nextInt();
				
		edge = new ArrayList[N+1];
		check = new boolean[N+1];
		
		for(int i = 1; i <= N; i++) {
			edge[i] = new ArrayList<>();
		}
		
		for(int i = 0; i < M; i++) {
			int v1 = sc.nextInt();
			int v2 = sc.nextInt();
			edge[v1].add(v2);
			edge[v2].add(v1);
		}
		
		bfs(1);
		System.out.println(ans);
	}
	
	static void bfs(int V) { 
		Queue<Integer> q = new LinkedList<>();
		q.add(V); check[V] = true;
		
		while(!q.isEmpty()) {
			int v = q.poll();
			for(int i = 0; i < edge[v].size(); i++) {
				int nv = edge[v].get(i);
				if(!check[nv]) {
					q.add(nv);
					check[nv] = true;
					ans++;
				}
			}			
		}
	}
	
}