notes49

# notes49 - CS251 Data Structures Class Notes Selection Sort...

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CS251: Data Structures 1/30/2006 - 2/3/2006 Class Notes Selection Sort and Priority Queues (PQ) With a list, O(n 2 ) is the best we can do to sort a PQ Heaps are a more efficient representation of a PQ Heaps are a special binary tree o A heap stores keys at its internal nodes and satisfies the following properties: Heap Order: for every internal node 'v' other than the root, key(v) ≤ key(parent(v)) Complete binary tree: let h be height. For i = 0... h-1, there are 2 i nodes of depth i. At depth h-1, the internal nodes are to the left of the external nodes. The heap must be left-full (Filled from left to right) To reorder the heap on insertion, we use a mehtod called upheap . Inserting into the heap is O(log 2 (n)). Removal is O(log 2 (n)) as well. To reorder the heap on removal, use a downheap by checking the children and swapping with the smallest child. On removal, you grab the last element in the heap and swap it with the one you want to remove. Therefore, an array is the best implementation of a heap. On a zero indexed array: o Left(i) = 2i + 1 o Right(i) = 2i + 2 Heap Sort Heap sort is O(n log(n)) instead of O(n 2 ).

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