문제는 다음과 같습니다.

https://www.acmicpc.net/problem/11724

　
import java.util.Scanner;

public class Test11724{
	static int N,M;
	static int visit[];
	static int graph[][];
	
	static void DFS(int x, int cnt) {
		visit[x]=cnt;
		for(int i=1; i<N+1; i++) {
			if(graph[x][i]==1 && visit[i]==0) {
				DFS(i,cnt);
			}
		}
	}
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		N = sc.nextInt();
		M = sc.nextInt();
		graph = new int[N+1][N+1];
		visit = new int[N+1];
		
		for(int i=0; i<M; i++) {
			int x = sc.nextInt();
			int y = sc.nextInt();
			graph[x][y]=graph[y][x]=1;
		}
		int cnt=1;
		for(int i=1; i<=N; i++) {
			if(visit[i]==0) {
				DFS(i, cnt);
				cnt++;
			}
		}
		System.out.println(cnt-1);
	}
	
	
}

　

결과는 다음과 같습니다.