leetcode--Implement Stack using Queues

mplement 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.

[java] view plain copy

1. class MyStack {  
2.     Queue<Integer> queue1 = new LinkedList<Integer>();  
3.     Queue<Integer> queue2 = new LinkedList<Integer>();  
4.     boolean turn = true;  
5.     // Push element x onto stack.  
6.     public void push(int x) {  
7.         if(turn){  
8.             queue1.add(x);  
9.         }else{  
10.             queue2.add(x);  
11.         }  
12.     }  
13.   
14.     // Removes the element on top of the stack.  
15.     public void pop() {  
16.         if(turn){  
17.             while(queue1.size()>1){  
18.                 queue2.add(queue1.poll());  
19.             }  
20.             queue1.poll();  
21.         }else{  
22.             while(queue2.size()>1){  
23.                 queue1.add(queue2.poll());  
24.             }  
25.             queue2.poll();  
26.         }  
27.         turn = !turn;  
28.     }  
29.   
30.     // Get the top element.  
31.     public int top() {  
32.         if(turn){  
33.             while(queue1.size()>1){  
34.                 queue2.add(queue1.poll());  
35.             }  
36.             return queue1.peek();  
37.         }else{  
38.             while(queue2.size()>1){  
39.                 queue1.add(queue2.poll());  
40.             }  
41.             return queue2.peek();  
42.         }  
43.     }  
44.   
45.     // Return whether the stack is empty.  
46.     public boolean empty() {  
47.         return queue1.isEmpty()&&queue2.isEmpty();  
48.     }  
49. } 

原文链接http://blog.csdn.net/crazy__chen/article/details/46581183