Palindromic numbers test

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

struct stackelem
{
    stackelem* next;
    char payload;
};

char temp [128];

class stack
{
private:
  stackelem *data;

public:

  stack () {data = NULL;}

  void reset() {
    while (data) pop();
  }

  bool empty() {
    return data ? false : true;
  }

  void push (char value)
  {
      stackelem* newelem = new stackelem;
      newelem->next = data;
      newelem->payload = value;
      data = newelem;
  }

  char pop()
  {
    if (data->payload)
    {
        char rvalue = data->payload;
        stackelem* old = data;
        data = data->next;
        delete old;
        return rvalue;
    }
    else {
        cerr << "Stack is empty \n";
        return 0;
    }
  }

};

int main ()
{
    stack st;
    fstream file ("input.txt", ios::in);
    if (!file)
    {
        cerr << "File not opened\n";
        return -1;
    }

    while (file)
    {
        st.reset();
        file.getline (temp, sizeof(temp));

        // clean noise characters

        int j = 0;
        for (int i = 0; temp[i]; i++)
            if ((temp[i] >= '0') && (temp[i] <= '9'))
                temp[j++] = temp[i];

        temp[j]=0;

        cout << "processing [" << temp << "] : ";

        // stack algorithm start

        if (j) {
            int k;

            for (k = 0; k < j/2; k++)
                st.push (temp[k]);

            if (j&1)
                k++;

            do
            {
                if ( st.pop() != temp[k++] )
                {
                    cout << "not symmetric [" << k << "]" << endl;
                    temp[k] = -1;
                    break;
                }

            } while (temp[k]);

            if (!temp[k])
                cout << "symmetric " << endl;
        }
    }

    return 0;
}