堆栈为空时抛出异常的问题.....队列/堆栈实现

Question

当两个堆栈都为空时我需要抛出一个异常，但我不知道该怎么写。

我必须实现一个有 2 个堆栈的队列！

这是主要的

#include "QueueFromStacks.h"

int main()
{
/*  THIS IS JUST TO SHOW YOU HOW #include <stack> WORKS
    stack<int> st1;
    stack<int> st2;
    cout << "Size before push:" << st2.size() << "\n";
    st2.push(2);
    st2.push(5);
    cout << "Size after two pushes:" << st2.size() << "\n";
    cout << st2.top() << "\n";
    st2.pop();
    cout << "Size of st2 after one pop:" << st2.size() << "\n";

    st1.push(st2.top());
    st2.pop();
    cout << "Size of st1:" <<st1.size()<< "   Size of st2:"<< st2.size();
*/
    QueueFromStacks<int> qfs;

    qfs.QueueFromStacks();

    qfs.enqueue(1);
    qfs.enqueue(2);
    qfs.enqueue(3);
    qfs.dequeue();
    cout  << "Queue Front : " << (qfs.front())<< endl;

// You have to implement QueuefromStack
// The logic of the queue remains the same(FIFO) but you have to use the two stacks to store your elements
// In the main program create a queuefromstack object and use your implemented methods to clearly show us what u did

    return 0;
}

头文件

    #ifndef QUEUEFROMSTACKS_H_
    #define QUEUEFROMSTACKS_H_
    
    #include <iostream>
    #include <stack>
    #include <string>
    
    using namespace std;
    
    class QueueEmptyException{
        public:
            QueueEmptyException();
            ~QueueEmptyException();
    
            string getMessage() { return "Queue is empty"; }
    };
    
    template <typename E>
    class QueueFromStacks
    {
        public:
            QueueFromStacks();
            ~QueueFromStacks();
    
            int size() const;
            bool empty() const;
            const E& front() const throw(QueueEmptyException);
            void enqueue (const E& e);
            void dequeue() throw(QueueEmptyException);
        private:
            stack<E> st1;
            stack<E> st2;
            int numElements;
    };
    #endif /* QUEUEFROMSTACKS_H_ */

实施

#include "QueueFromStacks.h"

template <typename E>
QueueFromStacks<E>::QueueFromStacks()
{
    numElements = 0;

}

template <typename E>
QueueFromStacks<E>::~QueueFromStacks()
{
    // TODO Auto-generated destructor stub
}

template <typename E>
int QueueFromStacks<E>::size() const
{
    return numElements;
}

template <typename E>
bool  QueueFromStacks<E>::empty() const
{
    return (size() == 0);
}

template <typename E>
const E& QueueFromStacks<E>::front() const
throw(QueueEmptyException)
{
    return st2.top();  // don't forget to check for empty and throw exception if it is empty.
}

template <typename E>
void QueueFromStacks<E>::enqueue (const E& e)
{
    st2.push(e);
    numElements++;
}

template <typename E>
void QueueFromStacks<E>::dequeue()
throw(QueueEmptyException)
{
    **// if both stacks are empty   // here i dont know what should i put as a throw condition  
    if (st1.empty() && st2.empty()) 
    {
        throw;
    }**

    // if the second stack is empty, move elements from the first stack to it
    if (st2.empty())
    {
        while (!st1.empty())
        {
            st2.push(st1.top());
            st1.pop();
        }
        // or make a call to swap(s1, s2)
    }

    // return the top item from the second stack
    int top = st2.top();
    st2.pop();
    numElements--;
}

Answer 1

你需要改变这个：

void QueueFromStacks<E>::enqueue (const E& e)
{
    st2.push(e);
    numElements++;
}

到这里：

void QueueFromStacks<E>::enqueue (const E& e)
{
    st1.push(e);
    numElements++;
}