C Program for Stack using linked list

/* STACK OPERATION USING LINK LIST */
#include<iostream.h>
#include<conio.h>
#include<alloc.h>
#include<stdio.h>
#include<process.h>
struct node
{
  int item;
  struct node *add;
}*h;
void push(struct node *a,int x)
{
  if(a==NULL)
  {
   a=(node *)malloc(sizeof(node));
   h=a;
   a->item=x;
   a->add=NULL;
   return;
  }
  while(a->add!=NULL)
  {
   a=a->add;
  }
  a->add=(node *)malloc(sizeof(node));
  if(a->add==NULL){cout<<"\n\aStack Overflow.";return;}
  a=a->add;
  a->item=x;
  a->add=NULL;
}
int pop(struct node * a)
{
 struct node *temp;
 if(a->add==NULL)h=NULL;
 if(a==NULL){cout<<"\n\aStack Underflow.";return 0;}
 while(a->add!=NULL)
  {
    temp=a;
    a=a->add;
  }
  temp->add=NULL;
  int x=a->item;
  a=NULL;
  return x;
}
void show(struct node *a)
{
   cout<<"List.:";
   while(a!=NULL)
   {
     cout<<" "<<a->item;
     a=a->add;
   }
}
void main()
{
  clrscr();
  cout<<"\nSTACK OPERATION USING LINK LIST\n";
  h=NULL;
  while(1)
  {
   cout<<"\n1.PUSH\t2.POP\t3.EXIT\n";
   char ch;
   int n;
   ch=getch();
   switch(ch)
   {
     case '1':
      cout<<"Enter no.:";
      cin>>n;
      push(h,n);
      show(h);
      break;
     case '2':
      cout<<"Poped.:"<<pop(h);
      show(h);
      break;
     case '3':
       exit(0);
   }
  }
}