LeetCode OJ：Implement Stack using Queues（队列实现栈）

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)

类似上一题的两个栈实现队列，但是有点不同，具体的见下面的注释：

 class Stack {
 public:
     // Push element x onto stack.
     void push(int x) {
         q1.push(x);
     }

     // Removes the element on top of the stack.
     void pop() {
         int tmp = q1.front();//注意这里的实现和双栈实现队列的方式是不相同的
         q1.pop();
         while(!q1.empty()){
             q2.push(tmp);
             tmp = q1.front();
             q1.pop();
         }
         swap(q1,q2);
     }

     // Get the top element.
     int top() {
         int tmp = q1.front();
         q1.pop();
         while(!q1.empty()){
             q2.push(tmp);
             tmp = q1.front();
             q1.pop();
         }
         q2.push(tmp);
         swap(q1, q2);//用swap即可，无须再向上次那样再倒回去
         return tmp;
     }

     // Return whether the stack is empty.
     bool empty() {
         return q1.empty();
     }
 private:
     queue<int> q1;
     queue<int> q2;
 };