Leetcode 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).

Update (2015-06-11):
The class name of the Java function had been updated to MyStack instead of Stack.

Solution 1:  O(n) push, others O(1)

 class MyStack {  
   // Push element x onto stack.  
   Deque<Integer> qu=new LinkedList<>();  
   public void push(int x) {  
     qu.offer(x);  
     int n=qu.size();  
     for (int i=0; i<n-1; i++) qu.offer(qu.poll());  
   }  
   // Removes the element on top of the stack.  
   public void pop() {  
     qu.poll();  
   }  
   // Get the top element.  
   public int top() {  
     return qu.peek();  
   }  
   // Return whether the stack is empty.  
   public boolean empty() {  
     return qu.size()==0;  
   }  
 }