18.用两个栈实现队列

template<typename T> 
class CQueue
{
public:
    CQueue() {}
    ~CQueue() {}

    void appendTail(const T& node);  // append a element to tail
    void deleteHead();               // remove a element from head 

private:
    stack<T> m_stack1;
    stack<T> m_stack2;
};

///////////////////////////////////////////////////////////////////////
// Append a element at the tail of the queue
///////////////////////////////////////////////////////////////////////
template<typename T> void CQueue<T>::appendTail(const T& element)
{
    // push the new element into m_stack1
    m_stack1.push(element);
} 

///////////////////////////////////////////////////////////////////////
// Delete the head from the queue
///////////////////////////////////////////////////////////////////////
template<typename T> void CQueue<T>::deleteHead()
{
    // if m_stack2 is empty, and there are some
    // elements in m_stack1, push them in m_stack2
    if(m_stack2.size() <= 0)
    {
        while(m_stack1.size() > 0)
        {
            T& data = m_stack1.top();
            m_stack1.pop();
            m_stack2.push(data);
        }
    }

    // push the element into m_stack2
    assert(m_stack2.size() > 0);
    m_stack2.pop();
}