add function is not working properly in Polynomial using linked list - linked-list

Question

I can't find any error in this code, please can anyone help me out with this.
please provide the whole code by debugging the given code.
I'm having a problem in adding two polynomials otherwise everything is fine.
I wasted my whole week in figuring out this bug but can't be able to find it.
so I will be happy if anyone can help me, please
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
struct Node
{
int coeff;
int exp;
struct Node *next;
} *poly = NULL, *poly2 = NULL;
void create()
{
struct Node *t, *last = NULL;
int num, i;
printf("Enter number of terms");
scanf("%d", &num);
printf("Enter each term with coeff and exp\n");
for (i = 0; i < num; i++)
{
t = (struct Node *)malloc(sizeof(struct Node));
scanf("%d%d", &t->coeff, &t->exp);
t->next = NULL;
if (poly == NULL)
{
poly = last = t;
}
else
{
last->next = t;
last = t;
}
}
}
void create2()
{
struct Node *t, *last = NULL;
int num, i;
printf("Enter number of terms");
scanf("%d", &num);
printf("Enter each term with coeff and exp\n");
for (i = 0; i < num; i++)
{
t = (struct Node *)malloc(sizeof(struct Node));
scanf("%d%d", &t->coeff, &t->exp);
t->next = NULL;
if (poly2 == NULL)
{
poly2 = last = t;
}
else
{
last->next = t;
last = t;
}
}
}
void Display(struct Node *p)
{
while (p)
{
printf("%dx%d +", p->coeff, p->exp);
p = p->next;
}
printf("\n");
}
long Eval(struct Node *p, int x)
{
long val = 0;
while (p)
{
val += p->coeff * pow(x, p->exp);
p = p->next;
}
return val;
}
void add(struct Node *p, struct Node *q)
{
struct Node *sum = (struct Node *)malloc(sizeof(struct Node));
while (p && q)
{
if (p->exp > q->exp)
{
sum->exp = p->exp;
sum->coeff = p->coeff;
p = p->next;
if (p)
{
sum->next = (struct Node *)malloc(sizeof(struct Node));
sum = sum->next;
}
}
else if (p->exp < q->exp)
{
sum->exp = q->exp;
sum->coeff = q->coeff;
q = q->next;
if (q)
{
sum->next = (struct Node *)malloc(sizeof(struct Node));
sum = sum->next;
}
}
else
{
sum->exp = p->exp;
sum->coeff = p->coeff + q->coeff;
q = q->next;
p = p->next;
if (p && q)
{
sum->next = (struct Node *)malloc(sizeof(struct Node));
sum = sum->next;
}
}
}
while (p)
{
sum->next = (struct Node *)malloc(sizeof(struct Node));
sum = sum->next;
sum->exp = p->exp;
sum->coeff = p->coeff;
p = p->next;
}
while (q)
{
sum->next = (struct Node *)malloc(sizeof(struct Node));
sum = sum->next;
sum->exp = q->exp;
sum->coeff = q->coeff;
q = q->next;
}
sum->next = NULL;
Display(sum);
}
int main()
{
create();
create2();
Display(poly);
//printf("\n");
Display(poly2);
//printf("\n");
add(poly, poly2);
return 0;
}