题解 | #【模板】栈#

import java.util.*;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        List<Integer> num = new ArrayList();
        // 注意 hasNext 和 hasNextLine 的区别
        if (in.hasNextInt()) { 
            int a = in.nextInt();
            while(in.hasNext()) {
                String t = in.nextLine();
                if (t.contains("top")) {
                    if(num.size()==0){
                        System.out.println("error");
                    }else{
                        System.out.println(num.get(num.size() - 1));
                    }
                } else if (t.contains("pop")) {
                    if(num.size()==0){
                        System.out.println("error");
                    }else{
                        System.out.println(num.get(num.size() - 1));
                        num.remove(num.size()-1);
                    }
                } else if(t.contains("push")){
                    Pattern pattern = Pattern.compile("(\\d+)"); // 匹配一个或多个数字
                    Matcher matcher = pattern.matcher(t);
                    if (matcher.find()) {
                        String numberStr = matcher.group(1); // 获取第一个捕获组的内容，即数字字符串
                        int number = Integer.parseInt(numberStr); // 将字符串转换为整数
                        num.add(number);
                    }
                }
            }
        }
    }
}