225. Implement Stack using Queues

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).

使用队列代替栈

class MyStack {
    // Push element x onto stack.
    private Queue<Integer> a=new LinkedList<>();
    public void push(int x) {
        a.add(x);
        for(int i=0;i<a.size()-1;i++)
        {
            a.add(a.poll());
        }
    }

    // Removes the element on top of the stack.
    public void pop() {
        a.poll();
    }

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

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