Program to evaluate an expression entered in post-fix form in C++

#include <iostream.h>
#include <stdlib.h>
#include <math.h>
#include <ctype.h>
#include <conio.h>
const int MAX = 50 ;
class postfix
{
int stack[MAX] ;
int top, nn ;
char *s ;
public :
postfix( ) ;
void setexpr ( char *str ) ;
void push ( int item ) ;
int pop( ) ;
void calculate( ) ;
void show( ) ;
} ;
postfix :: postfix( )
{
top = -1 ;
}
void postfix :: setexpr ( char *str )
{
s = str ;
}
void postfix :: push ( int item )
{
if ( top == MAX - 1 )
cout << endl << "Stack is full" ;
else
{
top++ ;
stack[top] = item ;
}
}
int postfix :: pop( )
{
if ( top == -1 )
{
cout << endl << "Stack is empty" ;
return NULL ;
}
int data = stack[top] ;
top-- ;
return data ;
}
void postfix :: calculate( )
{
int n1, n2, n3 ;
while ( *s )
{
if ( *s == ' ' || *s == '\t' )
{
s++ ;
continue ;
}
if ( isdigit ( *s ) )
{
nn = *s - '0' ;
push ( nn ) ;
}
else
{
n1 = pop( ) ;
n2 = pop( ) ;
switch ( *s )
{
case '+' :
n3 = n2 + n1 ;
break ;
case '-' :
n3 = n2 - n1 ;
break ;
case '/' :
n3 = n2 / n1 ;
break ;
case '*' :
n3 = n2 * n1 ;
break ;
case '%' :
n3 = n2 % n1 ;
break ;
case '$' :
n3 = pow ( n2 , n1 ) ;
break ;
default :
cout << "Unknown operator" ;
exit ( 1 ) ;
}

push ( n3 ) ;
}
s++ ;
}
}
void postfix :: show( )
{
nn = pop ( ) ;
cout << "Result is: " << nn ;
}

void main( )
{
clrscr();
char expr[MAX] ;
cout << "\nEnter postfix expression to be evaluated : " ;
cin.getline ( expr, MAX ) ;
postfix q ;
q.setexpr ( expr ) ;
q.calculate( ) ;
q.show( ) ;
getch();
}