앞만 보는 사람

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

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

public class Main {

    static int N, M, answer = 0;
    static boolean[] visited;
    static boolean[][] graph;

    public static void dfs(int idx) {
        visited[idx] = true;

        for (int i = 1; i <= N; i++) {
            if (graph[idx][i] && !visited[i]) {
                dfs(i);
            }
        }
    }
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer st = new StringTokenizer(br.readLine());

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

        graph = new boolean[N + 1][N + 1];
        visited = new boolean[N + 1];

        for (int i = 1; i <= M; i++) {
            st = new StringTokenizer(br.readLine());
            int u = Integer.parseInt(st.nextToken());
            int v = Integer.parseInt(st.nextToken());

            graph[u][v] = true;
            graph[v][u] = true;

        }

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

        System.out.print(answer);
    }
}

핵심

1. DFS를 언제 수행할지

2. graph, 재귀