LeetCode 225. Implement Stack using Queues 用队列实现栈

Question:

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).

Queue类：

实现： Queue <E> 自定义队列名称 = new LinkedList<E> ();

如： Queue <Integer> tmp = new LinkedList<> ();

注意： Queue <Integer> tmp = new Queue<> (); 会报错，因为Queue是一个抽象类，不能被实例化。

队列方法       等效方法
offer(e)      offer(e)/offerLast(e)  //进队列，将元素加入队列末尾
poll()        poll()/pollFirst()  //获取队列头的元素并移除
peek()        peek()/peekFirst()  //获取队列头的元素

	      isEmpty() //判断是否为空

参考：https://blog.csdn.net/ls5718/article/details/51700503

class MyStack {

    /** Initialize your data structure here. */
    public MyStack() {

    }
     Queue <Integer> data = new LinkedList<> ();
    // Queue <Integer> tmp = new Queue<> (); queue是抽象类不能被实例化
    Queue <Integer> tmp = new LinkedList<> ();
    /** Push element x onto stack. */
    public void push(int x) {
        tmp.offer(x);
        while (!data.isEmpty())
        {
            tmp.offer(data.poll());
        }
        while (!tmp.isEmpty())
        {
            data.offer(tmp.poll());
        }
    }

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

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

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

/**
* Your MyStack object will be instantiated and called as such:
* MyStack obj = new MyStack();
* obj.push(x);
* int param_2 = obj.pop();
* int param_3 = obj.top();
* boolean param_4 = obj.empty();
*/

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LeetCode 225. Implement Stack using Queues 用队列实现栈

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