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Heaps 3/19/14
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Heaps
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© 2014 Goodrich, Tamassia, Goldwasser
Presentation for use with the textbook Data Structures and Algorithms in Java, 6th edition, by M. T. Goodrich, R. Tamassia, and M. H. Goldwasser, Wiley, 2014
Heaps 2
Recall Priority Queue ADT
q A priority queue stores a collection of entries
q Each entry is a pair (key, value)
q Main methods of the Priority Queue ADT n insert(k, v)
inserts an entry with key k and value v
n removeMin() removes and returns the entry with smallest key
q Additional methods n min()
returns, but does not remove, an entry with smallest key
n size(), isEmpty()
q Applications: n Standby flyers n Auctions n Stock market
© 2014 Goodrich, Tamassia, Goldwasser
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Recall PQ Sorting q We use a priority queue
n Insert the elements with a series of insert operations
n Remove the elements in sorted order with a series of removeMin operations
q The running time depends on the priority queue implementation: n Unsorted sequence gives
selection-sort: O(n2) time n Sorted sequence gives
insertion-sort: O(n2) time
q Can we do better?
Algorithm PQ-Sort(S, C) Input sequence S, comparator C for the elements of S Output sequence S sorted in increasing order according to C P ← priority queue with
comparator C while ¬S.isEmpty ()
e ← S.remove (S. first ()) P.insert (e, e)
while ¬P.isEmpty() e ← P.removeMin().getKey() S.addLast(e)
© 2014 Goodrich, Tamassia, Goldwasser
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Heaps q A heap is a binary tree storing
keys at its nodes and satisfying the following properties:
q Heap-Order: for every internal node v other than the root, key(v) ≥ key(parent(v))
q Complete Binary Tree: let h be the height of the heap n for i = 0, … , h - 1, there are 2i
nodes of depth i n at depth h - 1, the internal nodes
are to the left of the external nodes
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q The last node of a heap is the rightmost node of maximum depth
last node
© 2014 Goodrich, Tamassia, Goldwasser
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Height of a Heap q Theorem: A heap storing n keys has height O(log n)
Proof: (we apply the complete binary tree property) n Let h be the height of a heap storing n keys n Since there are 2i keys at depth i = 0, … , h - 1 and at least one key
at depth h, we have n ≥ 1 + 2 + 4 + … + 2h-1 + 1
n Thus, n ≥ 2h , i.e., h ≤ log n
1
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2h-1
1
keys 0
1
h-1
h
depth
© 2014 Goodrich, Tamassia, Goldwasser
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Heaps and Priority Queues q We can use a heap to implement a priority queue q We store a (key, element) item at each internal node q We keep track of the position of the last node
(2, Sue)
(6, Mark) (5, Pat)
(9, Jeff) (7, Anna)
© 2014 Goodrich, Tamassia, Goldwasser
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Insertion into a Heap q Method insertItem of the
priority queue ADT corresponds to the insertion of a key k to the heap
q The insertion algorithm consists of three steps n Find the insertion node z
(the new last node) n Store k at z n Restore the heap-order
property (discussed next)
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insertion node
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z
z
© 2014 Goodrich, Tamassia, Goldwasser
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Upheap q After the insertion of a new key k, the heap-order property may be
violated q Algorithm upheap restores the heap-order property by swapping k
along an upward path from the insertion node q Upheap terminates when the key k reaches the root or a node
whose parent has a key smaller than or equal to k q Since a heap has height O(log n), upheap runs in O(log n) time
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7 9 6 z
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© 2014 Goodrich, Tamassia, Goldwasser
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Removal from a Heap q Method removeMin of
the priority queue ADT corresponds to the removal of the root key from the heap
q The removal algorithm consists of three steps n Replace the root key with
the key of the last node w n Remove w n Restore the heap-order
property (discussed next)
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last node
w
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9 w
new last node
© 2014 Goodrich, Tamassia, Goldwasser
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Downheap q After replacing the root key with the key k of the last node, the
heap-order property may be violated q Algorithm downheap restores the heap-order property by
swapping key k along a downward path from the root q Upheap terminates when key k reaches a leaf or a node whose
children have keys greater than or equal to k q Since a heap has height O(log n), downheap runs in O(log n) time
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© 2014 Goodrich, Tamassia, Goldwasser
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Updating the Last Node q The insertion node can be found by traversing a path of O(log n)
nodes n Go up until a left child or the root is reached n If a left child is reached, go to the right child n Go down left until a leaf is reached
q Similar algorithm for updating the last node after a removal
© 2014 Goodrich, Tamassia, Goldwasser
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Heap-Sort q Consider a priority
queue with n items implemented by means of a heap n the space used is O(n) n methods insert and
removeMin take O(log n) time
n methods size, isEmpty, and min take time O(1) time
q Using a heap-based priority queue, we can sort a sequence of n elements in O(n log n) time
q The resulting algorithm is called heap-sort
q Heap-sort is much faster than quadratic sorting algorithms, such as insertion-sort and selection-sort
© 2014 Goodrich, Tamassia, Goldwasser
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Array-based Heap Implementation q We can represent a heap with n
keys by means of an array of length n
q For the node at rank i n the left child is at rank 2i + 1 n the right child is at rank 2i + 2
q Links between nodes are not explicitly stored
q Operation add corresponds to inserting at rank n + 1
q Operation remove_min corresponds to removing at rank n
q Yields in-place heap-sort
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© 2014 Goodrich, Tamassia, Goldwasser
Java Implementation
© 2014 Goodrich, Tamassia, Goldwasser Heaps 14
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Java Implementation, 2
© 2014 Goodrich, Tamassia, Goldwasser Heaps 15
Java Implementation, 3
© 2014 Goodrich, Tamassia, Goldwasser Heaps 16
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Merging Two Heaps q We are given two two
heaps and a key k q We create a new heap
with the root node storing k and with the two heaps as subtrees
q We perform downheap to restore the heap-order property
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© 2014 Goodrich, Tamassia, Goldwasser
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q We can construct a heap storing n given keys in using a bottom-up construction with log n phases
q In phase i, pairs of heaps with 2i -1 keys are merged into heaps with 2i+1-1 keys
Bottom-up Heap Construction
2i -1 2i -1
2i+1-1
© 2014 Goodrich, Tamassia, Goldwasser
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Example
15 16 12 4 7 6 20 23
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15 16
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12 4
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20 23
© 2014 Goodrich, Tamassia, Goldwasser
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Example (contd.)
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20 23
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© 2014 Goodrich, Tamassia, Goldwasser
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Example (contd.)
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© 2014 Goodrich, Tamassia, Goldwasser
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Example (end)
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© 2014 Goodrich, Tamassia, Goldwasser
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Analysis q We visualize the worst-case time of a downheap with a proxy path
that goes first right and then repeatedly goes left until the bottom of the heap (this path may differ from the actual downheap path)
q Since each node is traversed by at most two proxy paths, the total number of nodes of the proxy paths is O(n)
q Thus, bottom-up heap construction runs in O(n) time q Bottom-up heap construction is faster than n successive insertions
and speeds up the first phase of heap-sort
© 2014 Goodrich, Tamassia, Goldwasser