Merge algorithm

[vekka]

Hi!
Could someone please help me to understand the use of merge algorithm(conce ptually) with linked lists. What I want to do in the linked list function is:
the function gets two inputs, i.e the pointers to the two smaller lists, then the function should able to merge these two together, and then return the head-pointer to this merged list. What happens in my case, is that I "lose" data when running it . My result is like this:

Even OR Odd numbers: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 ... 50 OK
Prime OR non Prime numbers: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15... OK
Even OR Prime numbers: 48 49 What happened here?
Odd OR Prime numbers: 49

Some sort of "Pseudocode ":

set_t *set_union(set_t *a, set_t *b) {

   set_node_t *head, *tail  (<---pointers to nodes)
  
   if (a < b) 
    head = tail = a->head
    a->head = a->head + 1

   if (a > b)
    head = tail = b->head
    b = b + 1

    while( a < end && b < end)
       if( a < b )
      tail->next = a->head
      a->head = a->head->next

      else
       tail->next = b->head
       b->head = b->head->next
     
       if b was exhausted before a
       if ( a->head != NULL)
           tail->next = a->head->next

       vice versa
       if(b->head != NULL) 
       tail->next = b->head->next


      return head

Will not this code actually return a merged list? or am I doing something wrong with the pointers? Thanks in advance!

[weaknessforcats]

A merge usually is a sorted merge of two lists.

you would
1) append the list to be merged to the final list.
2) sort the final list.
3) delete the list to be merged