5.4.2 Step 2: Insert

Following the search loop, q is the last non-null node examined, so it is the parent of the node to be inserted. The code below creates and initializes a new node as a child of q on side dir, and stores a pointer to it into n. Compare this code for insertion to that within <BST item insertion function 32>.

149. <Step 2: Insert AVL node 149> =
n = q->avl_link[dir] = 
  tree->avl_alloc->libavl_malloc (tree->avl_alloc, sizeof *n);
if (n == NULL)
  return NULL;

tree->avl_count++;
n->avl_data = item;
n->avl_link[0] = n->avl_link[1] = NULL;
n->avl_balance = 0;
if (y == NULL)
  return &n->avl_data;

This code is included in 146.

Exercises:

1. How can y be NULL? Why is this special-cased? [answer]