숫자고르기 - 2668번

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내 코드

package com.company;

import java.io.*;
import java.util.*;

public class Main {
    static int N;
    static boolean[] visited;
    static int[] arr;
    static HashSet<Integer> set = new HashSet<>();

    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        N = Integer.parseInt(br.readLine());

        arr = new int[N + 1];
        for (int i = 1; i <= N; i++) {
            arr[i] = Integer.parseInt(br.readLine());
        }
        visited = new boolean[N + 1];

        for (int i = 1; i <= N; i++) {
            Arrays.fill(visited, false);
            dfs(i, i, 1);
        }
        System.out.println(set.size());
        ArrayList<Integer> list = new ArrayList<>(set);
        Collections.sort(list);
        for (int i : list) {
            System.out.println(i);
        }
    }

    static void dfs(int first, int index, int count) {
        if (visited[index]) {
            return;
        }
        visited[index] = true;

        if (arr[index] == first) {
            for (int i = 0; i <= N; i++) {
                if (visited[i])
                    set.add(i);
            }
            return;
        }

        dfs(first, arr[index], count + 1);
    }
}