C语言结合两个双向链表

#include <stdio.h>
#include <stdlib.h>

struct node {
   int data;
   struct node *prev;
   struct node *next;
};

struct node *list = NULL;
struct node *list_last = NULL;

struct node *even = NULL;
struct node *even_last = NULL;

struct node *odd = NULL;
struct node *odd_last = NULL;

struct node *current = NULL;

//Create Linked List
void insert(int data) {
   // Allocate memory for new node;
   struct node *link = (struct node*) malloc(sizeof(struct node));

   link->data = data;
   link->prev = NULL;
   link->next = NULL;

   // If head is empty, create new list
   if(data%2 == 0) {
      if(even == NULL) {
         even = link;
         return;
      } else {
         current = even;

         while(current->next != NULL)
            current = current->next;

         // Insert link at the end of the list
         current->next = link; 
         even_last = link;
         link->prev = current;
      }
   } else {
      if(odd == NULL) {
         odd = link;
         return;
      } else {
         current = odd;

         while(current->next!=NULL)
            current = current->next;

         // Insert link at the end of the list
         current->next = link; 
         odd_last = link;
         link->prev = current;
      }
   }
}

//display the list
void print_backward(struct node *head) {
   struct node *ptr = head;

   printf("
[last] <=>");
   //start from the beginning
   while(ptr != NULL) {        
      printf(" %d <=>",ptr->data);
      ptr = ptr->prev;
   }

   printf(" [head]/n");
}

//display the list
void printList(struct node *head) {
   struct node *ptr = head;

   printf("
[head] <=>");
   //start from the beginning
   while(ptr != NULL) {        
      printf(" %d <=>",ptr->data);
      ptr = ptr->next;
   }

   printf(" [last]/n");
}

void combine() {
   struct node *link;

   list = even;
   link = list;

   while(link->next!= NULL) {
      link = link->next;
   }

   link->next = odd;
   odd->prev = link;

   // assign link_last to last node of new list

   while(link->next!= NULL) {
      link = link->next;
   }

   list_last = link;  
}

int main() {
   int i;

   for(i = 1; i <= 10; i++)
      insert(i);

   printf("Even : ");
   printList(even);

   printf("Even (R): ");
   print_backward(even_last);

   printf("Odd  : ");
   printList(odd);

   printf("Odd (R) : ");
   print_backward(odd_last);

   combine();

   printf("Combined List :/n");
   printList(list);

   printf("Combined List (R):/n");
   print_backward(list_last);

   return 0;
}