用栈实现队列和用队列实现栈

怎么用栈实现队列？
队列的特点是：先进先出
可以用两个栈实现，将栈A的栈顶元素出栈，再压入栈B。循坏该动作，直到A栈为空。这时栈B的栈顶元素就是队首元素。栈B中元素依次出栈即出队列。

import java.util.ArrayList;

public class MyQueue {
    private ArrayList<Integer> in;
    private ArrayList<Integer> out;

    public MyQueue() {
        in = new ArrayList<Integer>();
        out = new ArrayList<Integer>();
    }

    public void push(int x) {
        in.add(x);
    }

    public int pop() {
        if (out.isEmpty()) {
            int size = in.size();
            for (int i = 0; i < size; i++) {
                int v = in.remove(in.size() - 1);
                out.add(v);
            }
        }

        return out.remove(out.size() - 1);
    }

    public int peek() {
        if (out.isEmpty()) {
            int size = in.size();
            for (int i = 0; i < size; i++) {
                int v = in.remove(in.size() - 1);
                out.add(v);
            }
        }

        return out.get(out.size() - 1);
    }

    public boolean empty() {
        return in.isEmpty() && out.isEmpty();
    }
}

怎么用队列实现栈？
栈的特点是：先进后出
调用队列的方法，取出队首元素尾插在队尾，如此循坏等价于逆序了队列。这时候的队首即是栈顶。

import java.util.LinkedList;

class MyStack {
    private LinkedList<Integer> queue;

    /** Initialize your data structure here. */
    public MyStack() {
        queue = new LinkedList<>();
    }

    /** Push element x onto stack. */
    public void push(int x) {
        queue.addLast(x);
    }

    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        int size = queue.size();
        for (int i = 0; i < size - 1; i++) {
            int v = queue.pollFirst();
            queue.addLast(v);
        }

        return queue.pollFirst();
    }

    /** Get the top element. */
    public int top() {
        int size = queue.size();
        for (int i = 0; i < size - 1; i++) {
            int v = queue.pollFirst();
            queue.addLast(v);
        }

        int v = queue.pollFirst();
        queue.addLast(v);
        return v;
    }

    /** Returns whether the stack is empty. */
    public boolean empty() {
        return queue.isEmpty();
    }
}