Tree - List Problem

Write a recursive function treeToList(Node root) that takes an ordered binary tree and rearranges the internal pointers to make a circular doubly linked list out of the tree nodes.

/********************************************************
 ****      The Great Tree List Recursion Problem     ****
 *******************************************************/
struct node;

typedef struct node *nodeptr;

struct node {
    nodeptr left;
    int data;
    nodeptr right;
};

/*
 * given two list nodes, join them together so the second
 * immediately follows the first.
 */
void join(nodeptr a, nodeptr b)
{
    a->right = b;
    b->left = a;
}
/*
 * given two circular doubly linked lists, append them
 * and return the new list.
 */
nodeptr append(nodeptr a, nodeptr b)
{
    nodeptr aLast, bLast;

    if(a == NULL) return b;
    if(b == NULL) return a;
   
    aLast = a->left;
    bLast = b->left;

    join(aLast,b);
    join(bLast,a);

    return a;
}
/*
 * Convert given BST to linked list
 */
nodeptr treeToList(nodeptr root)
{
    nodeptr aList, bList;
    if(root == NULL)
        return NULL;
    else {
        aList = treeToList(root->left);
        bList = treeToList(root->right);
       
        root->left = root;
        root->right = root;
       
        aList = append(aList, root);
        aList = append(aList, bList);

        return aList;
    }
}
/*
 * print circular doubly linked list
 */
void printList(nodeptr head)
{
    nodeptr temp = head;
    if(head != NULL) {
        while(head->right != temp) {
            printf("%d ", head->data);
            head = head->right;
        }
        printf("%d ", head->data);
    }
    printf("\n");
}