C program to store a polynomial using linked list. Also perform addition and subtraction on two polynomials

#include <stdio.h>
#include <conio.h>
#include <malloc.h>
struct node
{
int num;
int coeff;
struct node *next;
};
struct node *start1 = NULL;
struct node *start2 = NULL;
struct node *start3 = NULL;
struct node *start4 = NULL;
struct node *last3 = NULL;
struct node *create_poly(struct node *);
struct node *display_poly(struct node *);
struct node *add_poly(struct node *, struct node *, struct node *);
struct node *sub_poly(struct node *, struct node *, struct node *);
struct node *add_node(struct node *, int, int);
int main()
{
int option;
clrscr();
do
{
 printf("\n******* MAIN MENU *******");
printf("\n 1. Enter the first polynomial");
printf("\n 2. Display the first polynomial");
 printf("\n 3. Enter the second polynomial");
 printf("\n 4. Display the second polynomial");
 printf("\n 5. Add the polynomials");
 printf("\n 6. Display the result");
 printf("\n 7. Subtract the polynomials");
 printf("\n 8. Display the result");
 printf("\n 9. EXIT");
 printf("\n\n Enter your option : ");
 scanf("%d", &option);
 switch(option)
 {
 case 1: start1 = create_poly(start1);
 break;
 case 2: start1 = display_poly(start1);
 break;
 case 3: start2 = create_poly(start2);
 break;
 case 4: start2 = display_poly(start2);
 break;
 case 5: start3 = add_poly(start1, start2, start3);
 break;
 case 6: start3 = display_poly(start3);
 break;
 case 7: start4 = sub_poly(start1, start2, start4);
 break;
 case 8: start4 = display_poly(start4);

break;
 }
}while(option!=9);
getch();
return 0;
}
struct node *create_poly(struct node *start)
{
struct node *new_node, *ptr;
int n, c;
printf("\n Enter the number : ");
scanf("%d", &n);
printf("\t Enter its coefficient : ");
scanf("%d", &c);
while(n != –1)
{
 if(start==NULL)
 {
 new_node = (struct node *)malloc(sizeof(struct node));
 new_node -> num = n;
 new_node -> coeff = c;
 new_node -> next = NULL;
 start = new_node;
 }
 else
 {
 ptr = start;
 while(ptr -> next != NULL)
 ptr = ptr -> next;
 new_node = (struct node *)malloc(sizeof(struct node));
 new_node -> num = n;
 new_node -> coeff = c;
 new_node -> next = NULL;
 ptr -> next = new_node;
 }
 printf("\n Enter the number : ");
 scanf("%d", &n);
 if(n == –1)
 break;
printf("\t Enter its coefficient : ");
 scanf("%d", &c);
}
return start;
}
struct node *display_poly(struct node *start)
{
struct node *ptr;
ptr = start;
while(ptr != NULL)
{
 printf("\n%d x %d\t", ptr -> num, ptr -> coeff);
 ptr = ptr -> next;
}
return start;
}
struct node *add_poly(struct node *start1, struct node *start2, struct node *start3)
{
struct node *ptr1, *ptr2;
int sum_num, c;

ptr1 = start1, ptr2 = start2;
while(ptr1 != NULL && ptr2 != NULL)
{
 if(ptr1 -> coeff == ptr2 -> coeff)
 {
 sum_num = ptr1 -> num + ptr2 -> num;
 start3 = add_node(start3, sum_num, ptr1 -> coeff);
 ptr1 = ptr1 -> next;
 ptr2 = ptr2 -> next;
 }
 else if(ptr1 -> coeff > ptr2 -> coeff)
 {
 start3 = add_node(start3, ptr1 -> num, ptr1 -> coeff);
 ptr1 = ptr1 -> next;
 }
 else if(ptr1 -> coeff < ptr2 -> coeff)
 {
 start3 = add_node(start3, ptr2 -> num, ptr2 -> coeff);
 ptr2 = ptr2 -> next;
 }
}
if(ptr1 == NULL)
{
 while(ptr2 != NULL)
 {
 start3 = add_node(start3, ptr2 -> num, ptr2 -> coeff);
 ptr2 = ptr2 -> next;
 }
}
if(ptr2 == NULL)
{
 while(ptr1 != NULL)
 {
 start3 = add_node(start3, ptr1 -> num, ptr1 -> coeff);
 ptr1 = ptr1 -> next;
 }
}
return start3;
}
struct node *sub_poly(struct node *start1, struct node *start2, struct node *start4)
{
struct node *ptr1, *ptr2;
int sub_num, c;
ptr1 = start1, ptr2 = start2;
do
{
 if(ptr1 -> coeff == ptr2 -> coeff)
 {
 sub_num = ptr1 -> num – ptr2 -> num;
 start4 = add_node(start4, sub_num, ptr1 -> coeff);
 ptr1 = ptr1 -> next;
 ptr2 = ptr2 -> next;
 }
 else if(ptr1 -> coeff > ptr2 -> coeff)
 {
 start4 = add_node(start4, ptr1 -> num, ptr1 -> coeff);
 ptr1 = ptr1 -> next;
 }
 else if(ptr1 -> coeff < ptr2 -> coeff
{
 start4 = add_node(start4, ptr2 -> num, ptr2 -> coeff);
 ptr2 = ptr2 -> next;
 }
}while(ptr1 != NULL || ptr2 != NULL);
if(ptr1 == NULL)
{
 while(ptr2 != NULL)
 {
 start4 = add_node(start4, ptr2 -> num, ptr2 -> coeff);
 ptr2 = ptr2 -> next;
 }
}
if(ptr2 == NULL)
{
 while(ptr1 != NULL)
 {
 start4 = add_node(start4, ptr1 -> num, ptr1 -> coeff);
 ptr1 = ptr1 -> next;
 }
}
return start4;
}
struct node *add_node(struct node *start, int n, int c)
{
struct node *ptr, *new_node;
if(start == NULL)
{
 new_node = (struct node *)malloc(sizeof(struct node));
 new_node -> num = n;
 new_node -> coeff = c;
 new_node -> next = NULL;
 start = new_node;
}
else
{
 ptr = start;
 while(ptr -> next != NULL)
 ptr = ptr -> next;
 new_node = (struct node *)malloc(sizeof(struct node));
 new_node -> num = n;
 new_node -> coeff = c;
 new_node -> next = NULL;
 ptr -> next = new_node;
}
return start;
}