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.

Example:

MyStack stack = new MyStack();

stack.push(1);
stack.push(2);
stack.top(); // returns 2
stack.pop(); // returns 2
stack.empty(); // returns false

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).

java

双端队列.

class  {

    ArrayDeque<Integer> queue;

    
    public () {
       queue = new ArrayDeque<>();
    }

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

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

    /** Get the top element. */
    public int top() {
      return queue.getLast();
    }

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

java

单向队列.

class 
{
    Queue<Integer> queue;

    public ()
    {
        this.queue=new LinkedList<Integer>();
    }

    // Push element x onto stack.
    public void push(int x)
    {
       queue.add(x);
       for(int i=0;i<queue.size()-1;i++)
       {
           queue.add(queue.poll());
       }
    }

    // Removes the element on top of the stack.
    public int pop()
    {
        return queue.poll();
    }

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

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

参考

implement-stack-using-queues

225. implement stack using queues 参考

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