day6 - COP 3503 Computer Science II CLASS NOTES - DAY #6...

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COP 3503 – Computer Science II CLASS NOTES - DAY #6 Additional Data Structures Balancing Trees As search trees get large, it becomes important to ensure that the tree is balanced, otherwise the time required by the various tree operations (searching primarily) will increase to a worst case of O(N). Later in the term, we will examine several different variants of trees and see how they are balanced. Some trees require that balance be maintained by all operations on the tree while other trees allow balancing to occur only after the tree has become unbalanced to the point of requiring too much time for individual operations on the tree. Recall that a binary tree is height-balanced or simply balanced if the difference in height of both subtrees of any node is either zero or one. A perfectly balanced tree is one in which all leaf nodes are found on one or two levels. For example, a perfectly balanced binary tree consisting of 10,000 nodes, the height of this tree will be log(10,001) = 13.289 = 14. In practical terms, this means that if 10,000 elements are stored in a perfectly balanced tree, then at most 14 nodes will need to be checked to locate a specific element. This is a substantial difference when compared to the worst case of 10,000 elements in a list! Therefore, in trees which are to be used primarily for searching, it is worth the effort to either build the tree so that it is balanced or modify the existing tree so that it is balanced. Day 6 - 1 A binary tree is height-balanced (or simply balanced ) if the difference in height of both subtrees of any node in the tree is either zero or one. A tree is said to be perfectly balanced if it is balanced and all of the leaves are found on one or two levels.
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Hash Tables Hash functions are a specific case of a more general technique known as key-to- address transformations (KTA transformations). There are many different KTA transformation techniques possible. Figure 1, illustrates the hierarchy of KTA transformations. Figure 1 – Key-to-address transformation hierarchy. Distribution dependent transformations depend on at least approximate knowledge of the key values that will be expected. The benefits that can be gained by distribution dependent techniques depend on open-addressing, bucket size. file density, and the appropriateness of the transformation itself. For small bucket size and a good distribution algorithm, the improvement over randomizing transformations can be significant. On the other hand, the liabilities of distribution dependent transformations are major, since a change in the key distribution can cause these methods to generate many more collisions than a randomization would generate for the same data. A benefit of some distribution dependent KTA transforms is that they can allow for maintaining sequentiality. Such sequence maintaining transforms allow the addresses produced to increase with increasing value of the key. Serial access is made possible in this case. Otherwise, a direct
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day6 - COP 3503 Computer Science II CLASS NOTES - DAY #6...

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