Add two numbers represented by linked lists

#include<stdio.h> #include<stdlib.h>   /* Linked list node */ struct Node {     int data;     struct Node* next; };   /* Function to create a new node with given data */ struct Node *newNode(int data) {     struct Node *new_node = (struct Node *) malloc(sizeof(struct Node));     new_node->data = data;     new_node->next = NULL;     return new_node; }   /* Function to insert a node at the beginning of the Doubly Linked List */ void push(struct Node** head_ref, int new_data) {     /* allocate node */     struct Node* new_node = newNode(new_data);       /* link the old list off the new node */     new_node->next = (*head_ref);       /* move the head to point to the new node */     (*head_ref)    = new_node; }   /* Adds contents of two linked lists and return the head node of resultant list */ struct Node* addTwoLists (struct Node* first, struct Node* second) {     struct Node* res = NULL; // res is head node of the resultant list     struct Node *temp, *prev = NULL;     int carry = 0, sum;       while (first != NULL || second != NULL) //while both lists exist     {         // Calculate value of next digit in resultant list.         // The next digit is sum of following things         // (i)  Carry         // (ii) Next digit of first list (if there is a next digit)         // (ii) Next digit of second list (if there is a next digit)         sum = carry + (first? first->data: 0) + (second? second->data: 0);           // update carry for next calulation         carry = (sum >= 10)? 1 : 0;           // update sum if it is greater than 10         sum = sum % 10;           // Create a new node with sum as data         temp = newNode(sum);           // if this is the first node then set it as head of the resultant list         if(res == NULL)             res = temp;         else // If this is not the first node then connect it to the rest.             prev->next = temp;           // Set prev for next insertion         prev  = temp;           // Move first and second pointers to next nodes         if (first) first = first->next;         if (second) second = second->next;     }       if (carry > 0)       temp->next = newNode(carry);       // return head of the resultant list     return res; }   // A utility function to print a linked list void printList(struct Node *node) {     while(node != NULL)     {         printf("%d ", node->data);         node = node->next;     }     printf("\n"); }   /* Driver program to test above function */ int main(void) {     struct Node* res = NULL;     struct Node* first = NULL;     struct Node* second = NULL;       // create first list 7->5->9->4->6     push(&first, 6);     push(&first, 4);     push(&first, 9);     push(&first, 5);     push(&first, 7);     printf("First List is ");     printList(first);       // create second list 8->4     push(&second, 4);     push(&second, 8);     printf("Second List is ");     printList(second);       // Add the two lists and see result     res = addTwoLists(first, second);     printf("Resultant list is ");     printList(res);      return 0; }