Implementing Queue With Two Stacks - Dequeuing Test Issue? - c++

Question

I'm trying to implement a queue with two stacks for purposes of understanding both data structures a little better. I have the below, with the main function serving as a test:
#include <iostream>
#include <stack>
using namespace std;
template <class T>
class _Stack : public stack<T> {
public:
T pop(){
T tmp=stack::top();
stack::pop();
return tmp;
}
};
template <class T>
class QueueS {
public:
QueueS(){}
bool isEmpty() const{
return pool.empty();
}
void enqueue(const T& el){
while( !output.empty()) {
input.push(output.pop());
}
input.push(el);
}
T dequeue(){
while(!input.empty()){
output.push(input.pop());
}
return output.pop();
}
T firstElement(){
if(output.empty()) {
return NULL;
}
return output.top();
}
private:
_Stack<T> pool;
_Stack<T> input;
_Stack<T> output;
};
int main(){
QueueS<int> n_QueueS;
//fill the queue of integers 0-9
for(int i=0; i<10;i++)
n_QueueS.enqueue(i);
// add another number to the queue
n_QueueS.enqueue(50);
//retrieve the first element without removing it
cout<<"front of the queue: "<<n_QueueS.firstElement()<<endl;
// removing the first 5 elements from the queue
cout<<"deleting first five elements of the queue: ";
for(int i=0; i<5;i++)
cout<<n_QueueS.dequeue()<<" ";
//removing the remainder of the queue and displaying the result
//should see 5 6 7 8 9 50 - see nothing!
cout<<endl<<"deleting remainder of the queue: ";
while(!n_QueueS.isEmpty())
cout<<n_QueueS.dequeue()<<" ";
if(n_QueueS.isEmpty())
cout<<endl<<"Queue is now empty";
else
cout<<endl<<"Error in emptying the queue";
system("pause");
return 0;
}
It works pretty well thusfar. However, when I run my test, deleting the first five elements works fine, and they display fine. It displays the line "deleting first five elements of the queue:" followed by 0 1 2 3 4, as expected.
However, deleting the second half doesn't display the values after the text "deleting remainder of the queue" like the previous test case did. I'm assuming the problem is minor, but I can't locate it through debugging. Maybe I've overlooked something?
Any help would be greatly appreciated!

Answer 1

First of all, your empty check should be something like this:
bool isEmpty() const{
return input.empty() && output.empty();
}
in enqueue, just push to the input stack:
void enqueue(const T& el){
input.push(el);
}
in enqueue and dequeue, move input to output if output is empty:
T dequeue(){
if (output.empty())
while(!input.empty()){
output.push(input.pop());
}
// throw exception of output.empty() ??
return output.pop();
}
T firstElement(){
if (output.empty())
while(!input.empty()){
output.push(input.pop());
}
if(output.empty()) {
return T(0); // throw exception?
}
return output.top();
}