题解 | #CD6 由两个栈组成的队列#

使用两个栈，一个栈保存输入数据，另一个栈倒序保存原输入栈的数据

• 运行时间：1412ms，超过82.04% 用Java提交的代码
• 占用内存：29272KB，超过90.63% 用Java提交的代码

import java.util.*;
import java.io.*;

public class Main {
    Deque<Integer> input = new LinkedList<>();
    Deque<Integer> reverse = new LinkedList<>();
    
    public static void main(String[] args) throws IOException {
        Main mn = new Main();
        BufferedReader sc = new BufferedReader(new InputStreamReader(System.in));
        int row = Integer.parseInt(sc.readLine());
        for(int i = 0; i < row; i++) {
            String s = sc.readLine();
            switch (s) {
                case "peek":
                    mn.peek();
                    break;
                case "poll":
                    mn.poll();
                    break;
                default:
                    String[] ss = s.split(" ");
                    int n = Integer.valueOf(ss[1]);
                    mn.add(n);
                    break;
            }
        }
    }
    
    private void peek() {
        if (reverse.isEmpty()) transfer();
        System.out.println(reverse.peek());
    }
    
    private void poll() {
        if (reverse.isEmpty()) transfer();
        reverse.pop();
    }
    
    private void add(int n) {
        input.push(n);
    }
    
    private void transfer() {
        while(!input.isEmpty()) {
            reverse.push(input.pop());
        }
    }
    
}