Insertion of a single element into index i of an array list is an O(n) operation, because every element from the ith onward must be shifted backward to make room for the new element. Deletion is likewise O(n). Thus, random insertion of n elements into an array list is an O(n^2) operation, which is undesirable.

A TreeList is a tree-based list. Unlike a BST, where each node stores a key, a value, and pointers to its children, a TreeList node stores the value, pointers to children, and the size of the node's left subtree. This last value is used like a key for search, insertion, and deletion. The TreeList can thereby store a sequence of ordered elements that can be inserted into and deleted from without needing to update the index of every element. Like a BST, insertion and deletion into a balanced BST can both be accomplished in O(log n) time. This makes the TreeList a more effective choice than an array list when performing a large number of insertions and deletions. In exchange, the TreeList loses the cache locality and constant-time random access properties of the array list.

As with ropes, each node in a TreeList records the size of its left subtree. A TreeList differs from a rope in that a TreeList stores values in all nodes, whereas a rope stores values exclusively in the leaves. A TreeList is also intended to store values of any type, rather than just strings.

Operating on a TreeList is mostly similar to operating on a BST.

• Get: Given index i, perform binary search as you would with a BST. Whenever you descend to a node's right subtree, subtract that node's size_of_left_subtree + 1.
• Insertion: Similar to BST insertion. When descending to a node's left subtree, increment its size_of_left_subtree.
• Deletion: Similar to BST deletion. When descending to a node's left subtree, decrement its size_of_left_subtree.
• Rotation: Similar to BST rotation. Updates to size_of_left_subtree for each node involved can be computed from the relevant node's current size_of_left_subtree parameters, as well as the size of the former root's subtree.