Description

Implement the following operations of a stack using queues.

• push(x) – Push element x onto stack.
• pop() – Removes the element on top of the stack.
• top() – Get the top element.
• empty() – Return whether the stack is empty.

Notes:

• You must use only standard operations of a queue – which means only push to back, peek/pop from front, size, and is empty operations are valid.
• Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

問題描述

用queue實現stack的如下操作

• push(x)–元素入棧
• pop()–刪除並返回棧頂元素
• top()–返回棧頂元素
• empty()–判斷棧是否空

問題分析

push元素，將元素放入queue的尾部，將queue尾部之前的元素刪除並依次放入當前尾部之後

解法

class MyStack {
    /** Initialize your data structure here. */
    private LinkedList<Integer> q1 = new LinkedList<>();
    public MyStack
 
() {
        q1 = new LinkedList<Integer>();
    }

    /** Push element x onto stack. */
    public void push(int x) {
        q1.add(x);
        int size = q1.size();
        while(size > 1){
            q1.add(q1.remove());
            size--;
        }
    }

    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return q1.remove();
    }

    /** Get the top element. */
    public int top() {
        return q1.peek();
    }

    /** Returns whether the stack is empty. */
    public boolean empty() {
        return q1.isEmpty();
    }
}