백준 1068

package baekjoon.트리;

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

class Main1068 {

   static int[] leaf;
   static int N, R, root;
   static ArrayList<Integer>[] g;

   public static void main(String[] args) throws Exception {
      input();
      go();
   }

   private static void go() {
      for (int i = 0; i < N; i++) {
         if (g[i].contains(R)) {
            g[i].remove(g[i].indexOf(R));
         }
      }
      if (root != R) {
         dfs(root);
      }
      System.out.println(leaf[root]);
   }

   private static void dfs(int x) {
      if (g[x].isEmpty()) {
         leaf[x] = 1;
      }
      for (int y : g[x]) {
         dfs(y);
         leaf[x] += leaf[y];
      }
   }

   private static void input() throws IOException {
      BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
      StringTokenizer st;
      N = Integer.parseInt(br.readLine());
      g = new ArrayList[N];
      leaf = new int[N];
      for (int i = 0; i < N; i++) {
         g[i] = new ArrayList<>();
      }
      st = new StringTokenizer(br.readLine());
      for (int i = 0; i < N; i++) {
         int a = Integer.parseInt(st.nextToken());
         if (a == -1) {
            root = i;
            continue;
         }
         g[a].add(i);
      }
      R = Integer.parseInt(br.readLine());
   }
}