C program to merge to sorted linked list

/*
Let x = (x 1 ,x 2 , ... , x n ) and y = (y 1 , y 2 ,.... , y m ) be two doubly linked lists. Assume that in
each linked list, the nodes are in non-decreasing order of their data-field values. Write
C/C++ program to merge the two lists to obtain a new linked list z in which the nodes are
also in this order. Following the merge, x and y should represent empty lists because each
node initially in x or y is now in z. No additional nodes may be used
*/

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

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

int main()
{
    int l1,l2,i;
   
    printf("\nEnter total no. of node in list 1\n");
    scanf("%d",&l1);
    printf("\nEnter total no. of node in list 2\n");
    scanf("%d",&l2);

    struct node *list1=NULL;
    i=0;
    while(i<l1)
{
    if(list1==NULL)
  {
    list1=(struct node *) malloc(sizeof(struct node));
    printf("\nEnter the no. in list 1\n");
    scanf("%d",&list1->data);

    list1->next=NULL;
    list1->prv=NULL;
  }
     else
  {
    struct node *temp;
    temp=list1;
    while(temp->next!=NULL)
    temp=temp->next;
    struct node *newnode=(struct node *) malloc(sizeof(struct node));
    printf("\nEnter the no. in list 1\n");
    scanf("%d",&newnode->data);

         temp->next=newnode;
         newnode->next=NULL;
         newnode->prv=temp;
  }
 i++;
}

    struct node *list2=NULL;
    i=0;
    while(i<l2)
{
     if(list2==NULL)
  {
      list2=(struct node *) malloc(sizeof(struct node));
      printf("\nEnter the no. in list 2\n");
      scanf("%d",&list2->data);
      list2->next=NULL;
      list2->prv=NULL;
  }
      else
  {
      struct node *temp;
      temp=list2;
      while(temp->next!=NULL)
      temp=temp->next;
      struct node *newnode=(struct node *) malloc(sizeof(struct node));
      printf("\nEnter the no .in list 2\n");
      scanf("%d",&newnode->data);
      temp->next=newnode;
      newnode->next=NULL;
      newnode->prv=temp;
  }
 i++;
}
    printf("\nList 1\n");
    struct node *temp;
    temp=list1;
    while(temp!=NULL)
    {
    printf("\t%d",temp->data);
      temp=temp->next;
    }
    printf("\nList 2\n");
    temp=list2;
    while(temp!=NULL){
      printf("\t%d",temp->data);
      temp=temp->next;
    }

struct node *z=NULL;

while(list1!=NULL && list2!=NULL)
{
      if(list1->data < list2->data)
    {
      if(z==NULL)
      {
        z=list1;
        list1=list1->next;
        z->next=NULL;
        z->prv=NULL;
      }
      else
      {
        struct node *tt;
          tt=z;
          while(tt->next!=NULL)
          tt=tt->next;
          tt->next=list1;
          list1->prv=tt;
          list1=list1->next;
          tt->next->next=NULL;
      }
    }

else
{
    if(z==NULL)
      {
        z=list2;
        list2=list2->next;
        z->next=NULL;
        z->prv=NULL;
      }
      else
      {
       struct node *tt;
         tt=z;
         while(tt->next!=NULL)
         tt=tt->next;
         tt->next=list2;
         list2->prv=tt;
         list2=list2->next;
         tt->next->next=NULL;
      }
   }
}

while(list1!=NULL)
{
     if(z==NULL)
      {
        z=list1;
        list1=list1->next;
        z->prv=NULL;
      }
      else
      {
       struct node *tt;
        tt=z;
        while(tt->next!=NULL)
        tt=tt->next;
        tt->next=list1;
        list1->prv=tt;
        list1=list1->next;
        tt->next->next=NULL;
    }
}
while(list2!=NULL)
{
     if(z==NULL)
      {
        z=list2;
        list2=list2->next;
        z->prv=NULL;
      }
      else
      {
       struct node *tt;
         tt=z;
         while(tt->next!=NULL)
         tt=tt->next;
         tt->next=list2;
         list2->prv=tt;
         list2=list2->next;
         tt->next->next=NULL;
      }
}

    printf("\nmerged list\n");
    temp=z;
    while(temp!=NULL)
    {
      printf("\t%d",temp->data);
     temp=temp->next;
    }
return 0;
}
/*
output:


Enter total no. of node in list 1
4

Enter total no. of node in list 2
5

Enter the no. in list 1
1

Enter the no. in list 1
2

Enter the no. in list 1
3

Enter the no. in list 1
4

Enter the no. in list 2
5

Enter the no .in list 2
6

Enter the no .in list 2
7

Enter the no .in list 2
8

Enter the no .in list 2
9

List 1
    1    2    3    4
List 2
    5    6    7    8    9
merged list
    1    2    3    4    5    6    7    8    9
*/