Balanced Parenthesis by using Stacks

/*
 * C++ Program to Check for balanced parenthesis by using Stacks
 */
#include <iostream>
#include <conio.h>
using namespace std;
struct node
{
    char data;
    node *next;
}*p = NULL, *top = NULL, *save = NULL,*ptr;
void push(char x)
{
    p = new node;
    p->data = x;
    p->next = NULL;
    if (top == NULL)
    {
        top = p;
    }
    else
    {
        save = top;
        top = p;
        p->next = save;
    }
}
char pop()
{
    if (top == NULL)
    {
        cout<<"underflow!!";
    }
    else
    {
        ptr = top;
        top = top->next;
        return(ptr->data);
        delete ptr;
    }
}  
int main()
{
    int i;
    char c[30], a, y, z;
    cout<<"enter the expression:\n";
    cin>>c;
    for (i = 0; i < strlen(c); i++)
    {
	if ((c[i] == '(') || (c[i] == '{') || (c[i] == '['))
	{
            push(c[i]);
	}
	else 
	{
	    switch(c[i])
	    {
		case ')':
                         a = pop(); 
		         if ((a == '{') || (a == '['))
			 {
			     cout<<"invalid expr!!";
                             getch();
			 }   
			 break;
		case '}':
                         y = pop();
			 if ((y == '[') || (y == '('))
			 {
			     cout<<"invalid expr!!";
                             getch();
			 }   
			 break;
		case ']':
                         z = pop();
			 if ((z == '{') || (z == '('))
			 {
			     cout<<"invalid expr!!";
                             getch();
			 }   
			 break;
	    }
	}
    }
    if (top == NULL)
    {
	cout<<"balanced expr!!";
    }
    else
    {
	cout<<"string is not valid.!!";
    }
    getch();
}