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).

 1 public class LC_225_ImplementStackUsingOneQueue {
 2 
 3     private Queue<Integer> queue1;
 4     private int size ;
 5     public LC_225_ImplementStackUsingOneQueue() {
 6         queue1 = new LinkedList<>();
 7     }
 8 
 9     /**
10      * Push element x onto stack.
11      */
12     public void push(int x) {
13         queue1.offer(x);
14     }
15 
16     /**
17      * Removes the element on top of the stack and returns that element.
18      * stack 1 2 3 4  -> pop the 4
19      * queue 4 3 2 1  converts to 3214
20      */
21     public int pop() {
22         size = queue1.size() ;
23         for (int i = 0; i < size-1; i++) {
24             queue1.offer(queue1.poll()) ;
25         }
26         return queue1.poll();
27     }
28 
29     /**
30      * Get the top element.: peek
31      * stack 1 2 3 4  -> pop the 4
32      * queue 4 3 2 1  converts to 3214
33      */
34     public int top() {
35         size = queue1.size() ;
36         for (int i = 0; i < size-1; i++) {
37             queue1.offer(queue1.poll()) ;
38         }
39         int value = queue1.peek() ;
40         //first get the 4 then offer the 4 so that the original order is maintained.
41         queue1.offer(queue1.poll());
42         return value;
43     }
44 
45     /**
46      * Returns whether the stack is empty.
47      */
48     public boolean empty() {
49         return queue1.isEmpty();
50     }
51 }

重要假设，必须要时刻知道queue 的 size 如果不知道，则做不了。