BOJ 11724

package boj;

import java.util.ArrayList;
import java.util.Scanner;

/**
 * Created by mkhwang on 2021/04/17
 * Email : orange2652@gmail.com
 * Github : http://github.com/myeongkwonhwang
 */
public class Boj11724 {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int vertex = sc.nextInt();  //정점
        int edge = sc.nextInt();    //간선의 수
        boolean[] visited = new boolean[vertex + 1];

        //인접리스트로 변경
        ArrayList<Integer>[] adjacencyList = new ArrayList[vertex + 1];
        for (int i = 1; i <= vertex; i++) {
            adjacencyList[i] = new ArrayList<>();
        }

        for (int i = 0; i < edge; i++) {
            int x = sc.nextInt();
            int y = sc.nextInt();
            adjacencyList[x].add(y);
            adjacencyList[y].add(x);
        }

        int ans = 0;

        for (int i = 1; i <= vertex; i++) {
            if(visited[i] == false) {
                dfs(adjacencyList, visited, i);
                ans += 1;
            }
        }

        System.out.println(ans);
    }

    private static void dfs(ArrayList<Integer>[] adjacencyList, boolean[] visited, int i) {
        if(visited[i]) return;

        visited[i] = true;

        for (int y : adjacencyList[i]) {
            if(visited[y] == false)
                dfs(adjacencyList, visited, y);
        }

    }
}