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)

很简单一道题，如果用栈实现队列有点难度，还需要两个栈。用队列实现栈的话还是比较简单的，用一个队列就可以完成了。

 1 class Stack {
 2 public:
 3     queue<int> Queue1;
 4     int* back,*tmp;
 5     // Push element x onto stack.
 6     void push(int x) {
 7         Queue1.push(x);
 8
 9
10     }
11
12     // Removes the element on top of the stack.
13     void pop() {
14         back=&Queue1.back();
15         while(&Queue1.front()!=back){
16             tmp=&Queue1.front();
17             Queue1.pop();
18             Queue1.push(*tmp);
19         }
20         Queue1.pop();
21     }
22
23     // Get the top element.
24     int top() {
25         return Queue1.back();
26     }
27
28     // Return whether the stack is empty.
29     bool empty() {
30         return Queue1.empty();
31
32     }
33 };