insertion function

I have a homework problem where I have to:Write an insertion function that inserts an element at the first position in the list following an element storing a particular DATA item. I also have to use this function. I don't understand what the problem is asking. Can someone explain this problem to me in terms that are easier to understand?

#include "list.h"

LINK lookup(DATA c, LINK head)
{

if (head == NULL)
   return N开发者_如何学GoULL;
else if ( c == head -> d)
        return head;
     else 
        return (lookup(c, head -> next));
}

The question is asking you to write a function that finds a specified item in the list, and then inserts the new item in the list after this position.

So given the list

List = {Item1, Item2, Item3, Item4}

Your function List.Insert(Item2, Item5) (for example) should result in

List = {Item1, Item2, Item5, Item3, Item4}

You have a linked list. The list is composed of LINK structs (to be more precise, it appears the LINK type is typedef'd to be a pointer to the actual node structs. We don't have the definition in front of us, but presumably each LINK struct holds a DATA struct as well as the usual linking pointer(s).

Your task is to take the head-of-the-list LINK and a DATA struct and search through the list until you find the first LINK that has a DATA identical to the one you're given. Then, insert the new element into the list just before that LINK.

You are asked to find DATA c in the linked list (the function lookup( ) will do that for you), then insert a DATA item between c and the DATA item following c. If c is the last DATA item in the list, then you are to add your new DATA item as the final DATA item. If the list is empty, i.e. if lookup( ) returns NULL, then your new DATA item should be stored as the one and only DATA item in the list.

Why not post the relevant part of list.h so we can understand what the linked list looks like?

It would also be a good idea to post the entire lookup( ) function.