题解 | 【模板】队列

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 in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        int count = in.nextInt();
        in.nextLine();
        Queue queue = new Queue(count + 1);
        while ((count-- > 0) && in.hasNextLine()) {
            String line = in.nextLine();
            String[] arr = line.split(" ");
            String operation = arr[0];
            try {
                if (operation.equals("push")) {
                    queue.push(Integer.parseInt(arr[1]));
                } else if (operation.equals("pop")) {
                    System.out.println(queue.pop());
                } else if (operation.equals("front")) {
                    System.out.println(queue.front());
                } else {
                    System.out.println("其他的非法操作");
                }
            } catch (RuntimeException e) {
                //e.printStackTrace();
                System.out.println("error");
            }
        }

    }
}

class Queue {
    int[] arr;
    int index = -1;

    Queue() {
        arr = new int[16];
    }

    Queue(int n) {
        arr = new int[n];
    }

    public void push(int x) {
        if (index + 2 > arr.length) {
            throw new RuntimeException("超出队列长度");
        }
        arr[++index] = x;
    }

    public String front() {
        if (index == -1) {
            throw new RuntimeException("队列为空");
        }        
        return Integer.toString(arr[0]);
    }

    public String pop() {
        if (index == -1) {
            throw new RuntimeException("队列为空");
        }
        String top = Integer.toString(arr[0]);
        for (int i = 1; i <= index; ++i) {
            arr[i - 1] = arr[i];
        }
        index -= 1;

        return top;
    }
}