第一个题解

import java.util.*;
public class Main{
    static Stack<Integer> stack1 = new Stack<Integer>();
    static Stack<Integer> stack2 = new Stack<Integer>();
    public static void add(int a){
        stack1.push(a);
        if(stack2.isEmpty()){
            while(!stack1.isEmpty()){
                stack2.push(stack1.pop());
            }
        }
    }
    public static int poll(){
        if(stack2.isEmpty() && !stack1.isEmpty()){
            while(!stack1.isEmpty()){
                stack2.push(stack1.pop());
            }
        }
        if(stack2.isEmpty()) throw new RuntimeException("队列空");
        return stack2.pop();
    }
    public static int peek(){
        if(stack2.isEmpty() && !stack1.isEmpty()){
            while(!stack1.isEmpty()){
                stack2.push(stack1.pop());
            }
        }
        if(stack2.isEmpty()) throw new RuntimeException("队列空");
        return stack2.peek();
    }
    public static void main(String[] args){
        Scanner scan = new Scanner(System.in);
        int n = Integer.valueOf(scan.nextLine());
        for(;n>0;n--){
            String str = scan.nextLine();
            String[] splits = str.split(" ");
            if("add".equals(splits[0])){
                add(Integer.parseInt(splits[1]));
            }else if("peek".equals(splits[0])){
                System.out.println(peek());
            }else{
                poll();
            }
        }
    }
}