240614 숫자고르기

static int n;
static int[] numbers;
static boolean[] visited;
static ArrayList<Integer> cycleList;

public static void main(String[] args) throws IOException {
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
	
	n = Integer.parseInt(br.readLine());
	numbers = new int[n + 1];
	
	for (int i = 1; i <= n; i++) {
		numbers[i] = Integer.parseInt(br.readLine());
	}
	
	cycleList = new ArrayList<>();
	visited = new boolean[n + 1];
	
	for (int i = 1; i <= n; i++) {
		if (!visited[i]) {
			visited[i] = true;
			dfs(i, i);
			visited[i] = false;
		}
	}
	
	Collections.sort(cycleList);
	StringBuilder sb = new StringBuilder();
	sb.append(cycleList.size()).append("\n");
	for (int num : cycleList) {
		sb.append(num).append("\n");
	}
	
	System.out.print(sb.toString());
	br.close();
}

static void dfs(int start, int target) {
	if (!visited[numbers[start]]) {
		visited[numbers[start]] = true;
		dfs(numbers[start], target);
		visited[numbers[start]] = false;
	}
	
	if (numbers[start] == target) {
		cycleList.add(target);
	}
}

출처:https://www.acmicpc.net/problem/2668