[BOJ] 2668. 숫자 고르기

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.Scanner;

public class Main {
	static Scanner sc=new Scanner(System.in);
	static int N,cnt;
	static boolean visit[];
	static List<Integer> vertex[]; 
	static List<Integer> list=new ArrayList<Integer>();
	static List<Integer> answer=new ArrayList<Integer>();
	static int visitCnt; 
	static boolean flag;
	public static void main(String[] args) {
		init();
		for (int i = 1; i <= N ; i++) {
			visitCnt=0;
			list.clear();
			visit=new boolean[N+1];
			for (int j = 0; j < answer.size(); j++) {
				visit[answer.get(j)]=true;
			}
			flag=false;
			if(visit[i]) continue;
			if(!go(i)) continue;
			for (int j = 0; j < list.size(); j++) {
				answer.add(list.get(j));					
			}
		}
		System.out.println(answer.size());
		Collections.sort(answer);
		for (int i = 0; i < answer.size(); i++) {
			System.out.println(answer.get(i));			
		}
	}
	//싸이클 체크 함수.
	public static boolean go(int v) {
		if(visit[v]) {
			if(v==list.get(0)) {
				flag=true;
				return flag;
			}
			return flag;
		}
		visit[v]=true;
		list.add(v);
		visitCnt++;
		for (int i = 0; i < vertex[v].size(); i++) {
			int nv=vertex[v].get(i);
			go(nv);
		}
		return flag;
	}
	
	public static void init() {
		N=sc.nextInt();
		vertex=new List[N+1];
		for (int i = 1; i <= N; i++) {
			vertex[i]=new ArrayList<Integer>();
		}
		
		for (int i = 1; i <= N; i++) {
			int nv=sc.nextInt();
			vertex[i].add(nv);
			if(i==nv) {
				answer.add(i); 
				cnt++;
			}
		}
	}
}