LC.225. Implement Stack using Queues(use one queue)

https://leetcode.com/problems/implement-stack-using-queues/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).

public class LC_225_ImplementStackUsingOneQueue {

    private Queue<Integer> queue1;
    private int size ;
    public LC_225_ImplementStackUsingOneQueue() {
        queue1 = new LinkedList<>();
    }

    /**
     * Push element x onto stack.
     */
    public void push(int x) {
        queue1.offer(x);
    }

    /**
     * Removes the element on top of the stack and returns that element.
     * stack 1 2 3 4  -> pop the 4
     * queue 4 3 2 1  converts to 3214
     */
    public int pop() {
        size = queue1.size() ;
        for (int i = 0; i < size-1; i++) {
            queue1.offer(queue1.poll()) ;
        }
        return queue1.poll();
    }

    /**
     * Get the top element.: peek
     * stack 1 2 3 4  -> pop the 4
     * queue 4 3 2 1  converts to 3214
     */
    public int top() {
        size = queue1.size() ;
        for (int i = 0; i < size-1; i++) {
            queue1.offer(queue1.poll()) ;
        }
        int value = queue1.peek() ;
        //first get the 4 then offer the 4 so that the original order is maintained.
        queue1.offer(queue1.poll());
        return value;
    }

    /**
     * Returns whether the stack is empty.
     */
    public boolean empty() {
        return queue1.isEmpty();
    }
}

重要假设，必须要时刻知道queue 的 size 如果不知道，则做不了。