Transcript PPT

Priority Queue and Binary Heap
Neil Tang
02/12/2008
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Class Overview
 Priority queue
 Binary heap
 Heap operations: insert, deleteMin, de/increaseKey, delete,
buildHeap
 Application
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Priority Queue
A priority queue is a queue in which each element has a
priority and elements with higher priorities are supposed to
be removed before the elements with lower priorities.
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Possible Solutions
 Linked list: Insert at the front (O(1)) and traverse the list
to delete (O(N)).
 Linked list: Keep it always sorted. traverse the list to
insert (O(N)) and delete the first element (O(1)).
 Binary search tree
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Binary Heap
A binary heap is a binary tree that is completely filled, with possible
exception of the bottom level and in which for every node X, the key in
the parent of X is smaller than (or equal to) the key in X.
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Binary Heap
 A complete binary tree of height h has between 2h and
2h+1 -1 nodes. So h = logN.
 For any element in array position i, its left child in position
2i and the right child is in position (2i+1), and the parent
is in i/2.
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Insert 14
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Insert (Percolate Up)
Time complexity: O(logN)
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deleteMin
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deleteMin
(Percolate Down)
Time complexity: O(logN)
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Other Operations
 decreaseKey(p,)
 increaseKey(p, )
 delete(p)?
 delete(p)=decreaseKey(p,)+deleteMin()
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buildHeap
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buildHeap
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buildHeap
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buildHeap
 Theorem: For the perfect binary tree of height 2h+1-1
nodes the sum of the heights of the nodes is 2h+1-1-(h+1).
 Time complexity: 2*(2h+1-1-(h+1)) = O(N).
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Applications
 Problem: find the kth smallest element.
 Algorithm: buildHeap, then deleteMin k times.
 Time complexity: O(N+klogN) = O(NlogN).
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Applications
 Problem: find the kth largest element.
 Algorithm: buildHeap with the first k elements, check the
rest one by one. In each step, if the new element is larger,
deleteMin and insert the new one.
 Time complexity: O(k+(N-k)logk) = O(NlogN).
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