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).

题目大意：用队列，实现栈。

class MyStack {

    List<Integer> stack = new ArrayList<>();

    // Push element x onto stack.

    public void push(int x) {

        stack.add(x);

    }

    // Removes the element on top of the stack.

    public void pop() {

        if(!empty()){

            stack.remove(stack.size()-1);

        }

    }

    // Get the top element.

    public int top() {

        if(!empty()){

            return stack.get(stack.size()-1);

        }

        return -1;

    }

    // Return whether the stack is empty.

    public boolean empty() {

        return stack.size()==0;

    }

}