# Combine Two Doubly Linked Lists

## Implementation

Implementation of this algorithm is given below −

```#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;

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

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

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

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

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

// Insert link at the end of the list
}
}
}

//display the list

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

}

//display the list

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

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

void combine() {

list = even;

}

// assign link_last to last node of new list

}

}

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;
}
```

## Output

Output of the program should be −

```Even :
[head] <=> 2 <=> 4 <=> 6 <=> 8 <=> 10 <=> [last]
Even (R):
[last] <=> 10 <=> 8 <=> 6 <=> 4 <=> 2 <=> [head]
Odd  :
[head] <=> 1 <=> 3 <=> 5 <=> 7 <=> 9 <=> [last]
Odd (R) :
[last] <=> 9 <=> 7 <=> 5 <=> 3 <=> 1 <=> [head]
Combined List :

[head] <=> 2 <=> 4 <=> 6 <=> 8 <=> 10 <=> 1 <=> 3 <=> 5 <=> 7 <=> 9 <=> [last]
Combined List (R):

[last] <=> 9 <=> 7 <=> 5 <=> 3 <=> 1 <=> 10 <=> 8 <=> 6 <=> 4 <=> 2 <=> [head]
```