题解 | #【模板】队列#

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayDeque;
import java.util.Deque;
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) throws IOException {
        BufferedReader br  = new BufferedReader(
            new InputStreamReader(System.in)
        );
        StringBuilder sb = new StringBuilder();
        int size = Integer.parseInt(br.readLine());
        Deque<String> queue = new ArrayDeque<>();
        for (int i = 0; i < size; i++) {

            String str = br.readLine();
            if (str.equals("pop")) {
                if (queue.isEmpty()) {
                    sb.append("error").append("\n");
                    continue;
                }
                sb.append(queue.remove()).append("\n");
            } else if (str.equals("front")) {
                if (queue.isEmpty()) {
                    sb.append("error").append("\n");
                    continue;
                }
                sb.append(queue.element()).append("\n");
            } else {
                String str1 = str.substring(5);
                queue.add(str1);
            }
        }
        System.out.println(sb);
    }
}