сокращение строки

#include <iostream>
#include <string>
using namespace std;

struct IElem
{
   char value;
   IElem *next;
};

class IStack
{
   IElem *pbeg, *pend;
   int Size = 0;
public:
   IStack()
   {
     pbeg = NULL;
   }
   int get_size()
   {
     return Size;
   }
   char top();
   void push(int val);
   char pop();
};

void IStack::push(int val)
{
   IElem *ptr = new IElem;
   ptr->value = val;
   ptr->next = pbeg;
   pbeg = ptr;
   Size++;
}

char IStack::pop()
{
   if (!pbeg)
    return -1;
   IElem *ptr = pbeg;
   int val = pbeg->value;
   pbeg = pbeg->next;
   Size--;
   delete ptr;
   return val;
}

char IStack::top()
{
   if (!pbeg)
    return ',';
   char val = pbeg->value;
   return val;
}

int main()
{
  IStack stack;
  string s;

  cin >> s;
  int dlina = s.length(), i = 1;

  stack.push(s[0]);

  while (i < dlina)
{
  char a = stack.top();
    if (a == s[i])
      stack.pop();
    else if (a != s[i] || a == ',')
      stack.push(s[i]);
  i++;
}

 int elements = stack.get_size();
 if (elements == 0)
  cout << " ";
 else
  {
    char *arr = new char[elements];
    for (int i = elements - 1; i >= 0; i--)
      arr[i] = stack.pop();
    for (int i = 0; i < elements; i++)
      cout << arr[i];
  }
}