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# ps4sol - Introduction to Algorithms October 23, 2006...

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Unformatted text preview: Introduction to Algorithms October 23, 2006 Massachusetts Institute of Technology 6.046J/18.410J Professors Erik Demaine and Madhu Sudan Problem Set 4 Solutions Problem Set 4 Solutions Problem 4-1. AVL Trees. An AVL tree is a binary search tree that is height balanced , i.e., for each node x , the heights of the left and right subtrees of x differ by at most 1 . To implement an AVL tree, we maintain an extra field in each node: h [ x ] is the height of the node x . Our first goal is to prove that BST-SEARCH runs in O (lg n ) time on an AVL tree. (a) Prove that an AVL tree with n nodes has height Θ(lg n ) . ( Hint: Prove that an AVL tree of height h has at least F h nodes, where F h is the h th Fibonacci number.) Solution: Let N h denote the smallest possible number of nodes in an AVL tree of height h . We shall show by induction that N h ≥ F h . Base: Clearly, N = 1 ≥ 0 = F , and N 1 = 2 ≥ 1 = F 1 . Induction step: Assume h ≥ 2 and that for all k < h , N k ≥ F k . Now, for any AVL tree of height h , the height of one of the two subtrees of the root must be h- 1 ; the other subtree of the root must have height at least h- 2 . Therefore, the number of nodes in the tree is at least N h- 1 + N h- 2 + 1 , which by the induction hypothesis is at least F h- 1 + F h- 2 . Since this is true for all AVL trees of height h , we have that N h ≥ F h- 1 + F h- 2 = F h . Thus N h ≥ F h for all h . Now recall that F i +2 ≥ φ i , where φ > 1 is the golden ratio (see CLRS Exercise 3.2-7). This gives us that for h ≥ 2 , N h ≥ φ h- 2 , or h ≤ log φ N h + 2 . Thus for n ≥ 4 , the height h of an AVL tree with n nodes satisfies h ≤ log φ N h + 2 ≤ log φ n + 2 , which implies that an AVL tree with n nodes has height O (lg n ) . Since any binary tree on n nodes has height at least lg n , we also have that an AVL tree with n nodes has height Ω(lg n ) . This proves that the height of an AVL tree on n nodes is Θ(lg n ) . Our remaining goal is to show how to insert a node into an AVL tree while maintaining the height- balance invariant. To insert a key into an AVL tree, we first place a node with that key in the appropriate place in the binary search tree order, using BST-INSERT. After this insertion, the tree may no longer be height balanced. Specifically, the heights of the left and right children of some node may differ by 2 . (b) Describe a procedure BALANCE ( x ) that takes a node x , whose left and right child subtrees are height balanced and have heights that differ by at most 2 , i.e., | h [ right [ x ]]- h [ left [ x ]] | ≤ 2 , and alters the subtree rooted at x to be height balanced. ( Hint: Use rotations.) 2 Problem Set 4 Solutions Solution: We sketch how BALANCE ( x ) works rather than giving pseudocode....
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## This note was uploaded on 01/20/2012 for the course CS 6.006 taught by Professor Erikdemaine during the Fall '08 term at MIT.

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ps4sol - Introduction to Algorithms October 23, 2006...

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