In CSC 321, we considered basic implementations of binary trees and binary search trees. The following classes are variations of the versions from last semester: TreeNode.java, BinaryTree.java, BinarySearchTree.java. The
BinaryTree class contains several methods that use a divide-and-conquer approach, including an O(N)
size method. The
BinarySearchTree class, which inherits from
BinaryTree, overrides the
contains methods with O(log N) decrease-and-conquer versions. It also contains inefficient O(N) implementations of
rank, which you will similarly replace with O(log N) decrease-and-conquer versions.
asList method is incomplete. Implement this method using a divide-and-conquer approach. In particular, start with an empty list and then recursively traverse the tree (using an inorder traversal), adding each data item to the list. Finally, return that list. Note that this method is used by the
toString method, so it should be possible to print the contents of a binary tree after you have completed the method.
size method uses divide-and-conquer recursion to traverse the tree and count the number of nodes. To avoid the O(N) cost of this approach, you are to modify
TreeNode so that in addition to storing the data value and subtrees, each node also stores the sizes of its subtrees (and those values are updated when either subtree is changed). With this size information embedded in each node, it becomes simple to determine the size of any subtree in a tree. For example, suppose
current referred to a node in a binary tree. if you knew that the left subtree of
current contained 12 nodes, and the right subtree of
current contained 14 nodes, then the tree rooted at
current would contain 12+14+1 = 27 nodes.
Note: all of the
TreeNode methods should remain O(1).
Once you have made your modifications, reimplement the
size method in
BinaryTree so that it utilizes the size information in nodes to determine the tree size in O(1) time.
select method in
BinarySearchTree is O(N), since it first converts the tree into a list (using your
asList method) and then accesses the corresponding index in that list. Alternatively, a decrease-and-conquer approach can be taken, similar to the approach used in the quick-select algorithm described in lectures. Consider the following binary search tree:
Suppose you want the rank-4 value in the tree. Since there are 3 values in the left subtree, the rank-4 value must be at the root. If you wanted the rank-3 value in the tree, it must be in the left subtree (and in fact is the rank-3 value in the left subtree). If you wanted the rank-5 value in the tree, it must be in the right subtree (and in fact is the rank-1 value in the right subtree).
You are to reimplement the
select operation using this decrease-and-conquer approach, which does work proportional to the height of the binary search tree. As long as the tree is relatively balanced, the
select operation will be O(log N).
Similarly, it is possible to utilize the size information in nodes to implement an O(log N)
rank operation. The
rank operation is the inverse of
select - it finds a specific value in the tree and returns its order rank. For example, the rank of "cubs" in the above binary search tree is 2, whereas the rank of "pirates" is 5. If the
rank method is called on a value that does not appear in the tree, it should return -1.
You are to reimplement the
rank method using a decrease-and-conquer approach, so that the amount of work is proportional to the height of the binary search tree.