Leetcode: 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.

Better Solution: Only push is O(n), others are O(1). Using one queue.

 1 class MyStack {
 2     // Push element x onto stack.
 3     Queue<Integer> queue;
 4     
 5     public MyStack() {
 6         this.queue = new LinkedList<Integer>();
 7     }
 8     
 9     public void push(int x) {
10         queue.offer(x);
11         for (int i=1; i<=queue.size()-1; i++) {
12             queue.offer(queue.poll());
13         }
14     }
15 
16     // Removes the element on top of the stack.
17     public void pop() {
18         queue.poll();
19     }
20 
21     // Get the top element.
22     public int top() {
23         return queue.peek();
24     }
25 
26     // Return whether the stack is empty.
27     public boolean empty() {
28         return queue.isEmpty();
29     }
30 }

就是两个queues, 轮流把除最后一个之外的都放入另一个queue中.

 1 class MyStack {
 2     // Push element x onto stack.
 3     Queue<Integer> q1;
 4     Queue<Integer> q2;
 5     
 6     public MyStack() {
 7         q1 = new LinkedList<Integer>();
 8         q2 = new LinkedList<Integer>();
 9     }
10     
11     public void push(int x) {
12         q1.offer(x);
13     }
14 
15     // Removes the element on top of the stack.
16     public void pop() {
17         if (q1.isEmpty()) return;
18         while (q1.size() > 1) {
19             q2.offer(q1.poll());
20         }
21         q1.poll();
22         q1 = q2;
23         q2 = new LinkedList<Integer>();
24     }
25 
26     // Get the top element.
27     public int top() {
28         if (q1.isEmpty()) return -1;
29         while (q1.size() > 1) {
30             q2.offer(q1.poll());
31         }
32         int res = q1.poll();
33         q1 = q2;
34         q1.offer(res);
35         q2 = new LinkedList<Integer>();
36         return res;
37     }
38 
39     // Return whether the stack is empty.
40     public boolean empty() {
41         return q1.isEmpty();
42     }
43 }