Deleting an AVL Node Step 4 - Rebalance - GNU libavl 2.0.3
Author: Ben Pfaff

5.5.4 Step 4: Rebalance |

Now we have to write code to rebalance when it becomes necessary. We'll use rotations to do this, as before. Again, we'll distinguish the cases on the basis of x's balance factor, where x is y's right child:

    175. <Step 4: Rebalance after AVL deletion 175> =
    struct avl_node *x = y->avl_link[1];
    if (x->avl_balance == -1)
      {
        <Left-side rebalancing case 1 in AVL deletion 176>
      }
    else
      {
        <Left-side rebalancing case 2 in AVL deletion 177>
      }

This code is included in 174.

##### Case 1: x has - balance factor

If x has a - balance factor, we handle rebalancing in a manner analogous to case 2 for insertion. In fact, we reuse the code. We rotate right at x, then left at y. w is the left child of x. The two rotations look like this:

    176. <Left-side rebalancing case 1 in AVL deletion 176> =
    struct avl_node *w;
    <Rotate right at x then left at y in AVL tree 161>
    pa[k - 1]->avl_link[da[k - 1]] = w;

This code is included in 175.

##### Case 2: x has + or 0 balance factor

When x's balance factor is +, the needed treatment is analogous to Case 1 for insertion. We simply rotate left at y and update the pointer to the subtree, then update balance factors. The deletion and rebalancing then look like this:

When x's balance factor is 0, we perform the same rotation, but the height of the overall subtree does not change, so we're done and can exit the loop with break. Here's what the deletion and rebalancing look like for this subcase:

    177. <Left-side rebalancing case 2 in AVL deletion 177> =
    y->avl_link[1] = x->avl_link[0];
    x->avl_link[0] = y;
    pa[k - 1]->avl_link[da[k - 1]] = x;
    if (x->avl_balance == 0)
      {
        x->avl_balance = -1;
        y->avl_balance = +1;
        break;
      }
    else
      x->avl_balance = y->avl_balance = 0;

This code is included in 175.

Exercises:

1. In <Step 4: Rebalance after AVL deletion 175>, we refer to fields in x, the right child of y, without checking that y has a non-null right child. Why can we assume that node x is non-null? [answer]

2. Describe the shape of a tree that might require rebalancing at every level above a particular node. Give an example. [answer]