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PQ, binary heaps G.Kamberova, Algorithms Priority Queue ADT Binary Heaps Gerda Kamberova Department of Computer Science Hofstra University

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PQ, binary heaps G.Kamberova, Algorithms Overview Priority Queue (PQ) ADT definition. Applications Implementation - binary heaps (heaps). Complexity analysis

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PQ, binary heaps G.Kamberova, Algorithms Priority Queue ADT Priority Queue (PQ) ADT is a dynamic set – with keys viewed as priorities –The keys could be compared, so we can identify the element with highest priority – supports insert delete element with highest priority retrieve element with highest priority (without delete) D epending on what, min or max, is selected as highest priority, we distinguish between min- and max-prioirity queues Historically, PQ means min-PQ. If x is a reference to a record, the operations on priority queue S are: – For max-priority queue: Max(S), DeleteMax(S), Insert(S,x) – For min-priority queue: Min(S), DeleteMin(S), Insert(S,x)

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PQ, binary heaps G.Kamberova, Algorithms Applications Job scheduling on a multiuser system – Queue (FIFO). Not a good choice. – PQ: greedy algorithm: let the smallest job run first. Keep a PQ of incoming jobs and efficiently retrieve the highest priority job to run. Minimizes avg time to wait Event-driven discrete-event system simulation: most prevalent application of PQ is in

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PQ, binary heaps G.Kamberova, Algorithms PQ ADT Implementation Linked list (sorted or unsorted)? Array sorted or unsorted? Binary search tree (BST)? – O(log n), – overkill, supports all dynamic set operations Heap –no linked lists –better suited than BST for PQ –Implementation, internally the structure is an array O(log n) in worst case. Insert is O(1) on average. Building PQ is in linear time, O(n)

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PQ, binary heaps G.Kamberova, Algorithms Binary Heap (Heap) Heap (binary heap) is a binary tree (BT) in which the following properties are satisfied: –HS [the heap structure property or heap shape]: the BT is completely filled at all levels except possibly on the very last, where it is filled from left to right –PODR[ the partial order heap order property]: the key value stored at any non-leaf node is >= the key values of its children. –no particular Note: –order between key values on the same level. –Max is at the root –when you go from a leaf, up a direct path, to the root, the keys are encountered in non-decreasing order. –the operations insert and delete may destroy the heap properties, so these operations should not terminate before restoring the heap properties

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PQ, binary heaps G.Kamberova, Algorithms BT Representation of a Max Heap Check HS and PORD properties 14 8 24 7 10 1 9 3 16

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PQ, binary heaps G.Kamberova, Algorithms Implementation of a Heap since a heap is almost a complete BT, it is easily stored in an array (no need to store pointers): 14 8 24 7 10 1 9 3 16 16 14 10 8 7 9 3 2 4 1 …

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PQ, binary heaps G.Kamberova, Algorithms Implementation of a Heap 14 8 24 7 10 1 9 3 16 16 14 10 8 7 9 3 2 4 1 … 1 2 3 4 5 6 7 8 9 10 Read left to right, fill top to bottom left to right Read top to bottom, left to right, fill left to right, start at index 1 Nodes are labeled left-to-right in level order (root first, the right-most leaf last), and this order defines the array indices.

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PQ, binary heaps G.Kamberova, Algorithms Implementation of the PQ operations with heaps Retrieve the maximum –Easy, just return the root – Insert/DeleteMax –more complicated. –Must preserve HS and PORD properties. Insert: –Input: a heap A of size n and an element referenced by x with key k to be inserted. –Output: A heap A of size n+1. –Where to place the new node? not much choice, must preserve HS. create a new node at index n+1 If we place the new key at that place the PROD may be destroyed. restore PORD by sifting up the new node until it falls in place i.e. where the new key will be in place

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PQ, binary heaps G.Kamberova, Algorithms Insertion in a max heap: Example Insert element with key 15 in the heap Create a new node as most right leaf in tree Propagate “hole” up until it falls in place 14 8 24 7 10 1 9 3 16 15 7 14

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PQ, binary heaps G.Kamberova, Algorithms Insertion in a heap Implementation –Create the (n+1)-st node, and sift the key upwards Heap_Insert(A, key, n) n = n+1 i = n // find the spot for the new key while (i > 1 && A(Parent[i]) < key ) A[i] = A[Parent(i)] i = Parent(i) A[i] = key Complexity:

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PQ, binary heaps G.Kamberova, Algorithms Delete in a heap Input: Heap A of size n; Output: Heap A of size n-1, the root key value is removed from the heap (may be returned as well) Since the heap HS must be preserved, the output heap is missing the right-most leaf of the input heap. The root value must be extracted, and the “hole” at the root must be filled in This suggest the following procedure: –copy the key from right-most leaf to root –delete right-most leaf –Restore PORD: heapify

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PQ, binary heaps G.Kamberova, Algorithms Delete in a heap Heap_Delete(A, n) A[1] = A[n] //copy last leaf in root n = n - 1 Heapify(A,1)

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PQ, binary heaps G.Kamberova, Algorithms Deletion from the heap Heap_Delete(A, n) A[1] = A[n] //copy last leaf in root n = n - 1 Heapify(A,1) Complexity: O(1)+ time to heapify 14 8 24 7 10 1 9 3 16 1

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PQ, binary heaps G.Kamberova, Algorithms Heapify Input: A rooted tree, such that A[i] is at the root, and the left and the right subtrees (if exist) are heaps. Output: The tree is made into a heap. Procedure: –Sift down the key value of the root until it falls in place. –a recursive procedure: compare key at the root with the keys of its children. If the root key >= of the children keys, stop, Otherwise, swap root key with the key of larger child, and proceed in hipifying now the subtree rooted at the node for which swap occured

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PQ, binary heaps G.Kamberova, Algorithms Heapify Hipify(A,i) : makes the subtree rooted at A[i] into a heap, provided that the trees rooted at A[Left(i)] and A[Right(i)] are heaps. Hipify(A,i) if A[i] is smaller than its children m = index of larger child swap A[i] and A[m] Hipify(A,m)

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PQ, binary heaps G.Kamberova, Algorithms Deletion from the heap: Example 14 8 24 7 10 1 9 3 16 1

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PQ, binary heaps G.Kamberova, Algorithms Deletion from the heap: Example Complexity: if we swap, iteratively, bounded by the heigh Complexity using recursive heapify? 14 8 24 7 10 9 3 1

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PQ, binary heaps G.Kamberova, Algorithms Complexity of Heapify Worst case: at each recursive call the subtree on which the call is made on a complete BT (all leaves at last level exist); the height of that tree must be one more than the height of the other subtree In a complete BT the number of leaves is equal half of the nodes If the worst-case tree has n nodes, they are distributed as follows Recurrence for Heapify n/3

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PQ, binary heaps G.Kamberova, Algorithms Constructing a heap Top-down: using HeapInsert, –successively insert nodes staring with an empty heap. –Time complexity is O(nlog n) Bottom-up: using Heapify –Construct an almost complete binary tree with n vertices and store the keys arbitrarily there. This is Theta(n) –Now going bottom up, from the most right node in the last internal level, heapify the trees rooted at the internal nodes. –This time can be shown to be linear O(n)

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PQ, binary heaps G.Kamberova, Algorithms Heapsort 1.Build a heap: O(n) or O(n log n) 1.Do n times extract_max n times: O(n log n) Total time O(n log n)

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