[백준]_2606_바이러스

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public class B2606_바이러스 {
	
	static ArrayList<Integer>[] a;
    static boolean[] visit;
    static int count;
    
	
	public static void main(String[] args) throws NumberFormatException, IOException {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		StringBuilder sb = new StringBuilder();
		StringTokenizer st;
		
		int n = Integer.parseInt(br.readLine());
		int m = Integer.parseInt(br.readLine());
		
		a = new ArrayList[n+1]; 
		visit = new boolean[n+1];
		
		for (int i=1; i<=n; i++) {
            a[i] = new ArrayList<Integer>();
        }
		// 연결 된 부분 정보 저장
		for(int i=0;i<m;i++) {
			st = new StringTokenizer(br.readLine());
			int start = Integer.parseInt(st.nextToken());
			int end = Integer.parseInt(st.nextToken());
			a[start].add(end);
			a[end].add(start);
		}
		count=0;
		// 1번부터 탐색
		search(1);
		System.out.println(count);
	}
	
	// 연결된 부분 탐색
	public static void search(int x) {  
        visit[x] = true;
        for (int y : a[x]) {
            if (visit[y] == false) {
            	count++;
                search(y);
            }
        }
    }

}