题解 | 【模板】序列操作

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

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        List<Integer> list = new ArrayList<>();
        while (n-- > 0) {
            int a = scan.nextInt();
            if (a == 1) {
                int num = scan.nextInt();
                list.add(num);
            } else if (a == 2 && !list.isEmpty()) {
                list.remove(list.size() - 1);
            } else if (a == 3) {
                int num = scan.nextInt();
                System.out.println(list.get(num));
            } else if (a == 4) {
                int index = scan.nextInt();
                int num = scan.nextInt();
                list.add(index+1, num);
            } else if (a == 5) {
                Collections.sort(list);
            } else if (a == 6) {
                list.sort(Collections.reverseOrder());
            } else if (a == 7) {
                System.out.println(list.size());
            } else if (a == 8) {
                List<String> list1 = new ArrayList<>();
                for (Integer b : list) {
                    list1.add(String.valueOf(b));
                }
                System.out.println(String.join(" ", list1));
            }
        }
    }
}