AVL tree node.
| Node | *top | The parent node. |
| Node | *left | The left subnode (with a lesser or equal key value). |
| Node | *right | The right subnode (with a greater or equal key value). |
| int | balance | The balance of the node. '-1' means that the left subtree is taller than the right one. '1' means that the right subtree is taller than the left one. '0' means that right and left subtrees of this node are equally tall. |
| T | value | The value associated with this node. |