用两个栈实现队列（C++ 和 Python 实现）

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

    void appendTail(const T& node);
    T deleteHead();     
};

#include <iostream>
#include <stack>
#include <exception>


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

    void appendTail(const T& node);
    T deleteHead();

private:
    std::stack<T> stack1;
    std::stack<T> stack2;
};

// Declare queue empty exception when try to pop when the queue is empty.
class QEmptyException: public std::exception {
    virtual const char* what() const throw()
    {
        return "Error: Queue is empty!";
    }
} popException;

// stack1 as input stack, stack2 as output stack
template <typename T>
void CQueue<T>::appendTail(const T& node)
{
    stack1.push(node);
}

template <class T>
T CQueue<T>::deleteHead()
{
    // Check if there are elements in stack2
    if (stack2.empty())
    {
        // Pop all the elements from stack1, then push them onto stack2
        if (stack1.size() > 0)
        {
            while (!stack1.empty())
            {
                T elem = stack1.top();
                stack1.pop();
                stack2.push(elem);
            }
        }
        else
            throw popException;
    }

    // Get the top element from stack2, then delete it from stack2
    T elem = stack2.top();
    stack2.pop();

    return elem;
}

template <typename T>
CQueue<T>::CQueue(void)
{
}

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

void unitest()
{
    CQueue<int> que;

    std::cout << "Push 1, 2, 3 successively into CQueue." << std::endl;
    que.appendTail(1);
    que.appendTail(2);
    que.appendTail(3);
    std::cout << "Pop the head of the queue: " << que.deleteHead() << std::endl;
    std::cout << "Pop the head of the queue: " << que.deleteHead() << std::endl;
    std::cout << "Push 4, 5, 6 successively into CQueue." << std::endl;
    que.appendTail(4);
    que.appendTail(5);
    que.appendTail(6);
    // Pop the rest elements in the queue, until the queue empty exception happens
    for (int i = 0; i < 4; ++i)
    {
        std::cout << "Pop the head of the queue: " << que.deleteHead() << std::endl;
    }

}

int main()
{
    unitest();

    return 0;
}

#!/usr/bin/python
# -*- coding: utf8 -*-

class CQueue:
    def __init__(self):
        self.stack1 = []
        self.stack2 = []

    def append_tail(self, elem):
        self.stack1.append(elem)

    def delete_head(self):
        if not self.stack2:
            if self.stack1:
                while self.stack1:
                    elem = self.stack1.pop()
                    self.stack2.append(elem)
            else:
                raise Exception("Queue is empty.")
            
        elem = self.stack2.pop()
        return elem


def unitest():
    # Create an instance of class CQueue
    que = CQueue()
    print "Push 1, 2, 3 successively into CQueue."
    for i in range(1, 4):
        que.append_tail(i)
    print "Pop the head of the queue:", que.delete_head()
    print "Pop the head of the queue:", que.delete_head()
    print "Push 4, 5, 6 successively into CQueue."
    for i in range(4, 7):
        que.append_tail(i)
    # Pop the rest elements in the queue
    for i in range(4):
        print "Pop the head of the queue:", que.delete_head()
        

if __name__ == ‘__main__‘:
    unitest()