# 13-1 Persistent dynamic sets

During the course of an algorithm, we sometimes find that we need to maintain past versions of a dynamic set as it is updated. We call such a set persistent. One way to implement a persistent set is to copy the entire set whenever it is modified, but this approach can slow down a program and also consume much space. Sometimes, we can do much better.

Consider a persistent set $S$ with the operations $\text{INSERT}$, $\text{DELETE}$, and $\text{SEARCH}$, which we implement using binary search trees as shown in Figure 13.8(a). We maintain a separate root for every version of the set. In order to insert the key $5$ into the set, we create a new node with key $5$. This node becomes the left child of a new node with key $7$, since we cannot modify the existing node with key $7$. Similarly, the new node with key $7$ becomes the left child of a new node with key $8$ whose right child is the existing node with key $10$. The new node with key $8$ becomes, in turn, the right child of a new root $r'$ with key $4$ whose left child is the existing node with key $3$. We thus copy only part of the tree and share some of the nodes with the original tree, as shown in Figure 13.8(b).

Assume that each tree node has the attributes $key$, $left$, and $right$ but no parent. (See also Exercise 13.3-6.)

a. For a general persistent binary search tree, identify the nodes that we need to change to insert a key $k$ or delete a node $y$.

b. Write a procedure $\text{PERSISTENT-TREE-INSERT}$ that, given a persistent tree $T$ and a key $k$ to insert, returns a new persistent tree $T'$ that is the result of inserting $k$ into $T$.

c. If the height of the persistent binary search tree $T$ is $h$, what are the time and space requirements of your implementation of $\text{PERSISTENT-TREE-INSERT}$? (The space requirement is proportional to the number of new nodes allocated.)

d. Suppose that we had included the parent attribute in each node. In this case, $\text{PERSISTENT-TREE-INSERT}$ would need to perform additional copying. Prove that $\text{PERSISTENT-TREE-INSERT}$ would then require $\Omega(n)$ time and space, where $n$ is the number of nodes in the tree.

e. Show how to use red-black trees to guarantee that the worst-case running time and space are $O(\lg n)$ per insertion or deletion.

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