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Lecture 7 Heaps and Priority Queues

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Motivating Example 3 jobs have been submitted to a printer, the jobs have sizes 100, 10, 1 page. Average waiting time for FIFO service, (100+110+111)/3 = 107 time units Average waiting time for shortest job first service, (1+10+111)/3 = 36.67 time units Need to have a queue which does insert and deletemin Priority Queue

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Common Implementation Linked list Insert in O(1) Find the minimum element in O(n), thus deletion is O(n) Search Tree (AVL tree) Insert in O(log n) Delete in O(log n) Search Tree is an overkill as it does many other operations

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Heaps Almost complete binary tree All levels are complete except the lowest one (book says complete binary tree, I will define complete and almost complete whenever I use them) Note down example from the board If such a tree has height h, then it has between 2 h-1 + 2 and 2 h + 1 nodes. Thus h is O(log n) Mistake in the expression in your book

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Value of an element at a node is less than or equal to that of its descendants. Note down example from the board Mistake in your book Section 6.1, 6.2, 6.3 in book

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Array implementaion We start from position 1 in the array. The first element contains the root Left child of element at position j is at position 2j, right child is at position 2j + 1 Parent of element at position j is at j/2 Need to know the size of the heap. Would you recommend this implementation for any binary tree? Note down example from the board.

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Insertion Find an empty position in the heap If some nodes have one child, then the empty position is the other child If all nodes have 0 or 2 children, then the empty position is the left child of a leaf. Insert the element there. Let the element be inserted at position j. If the parent of the element is more than the element, then interchange the parent and child Interchange elements at j/2 and j.

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If necessary, interchange elements at j/2 with j/2 /2 And so on till we reach the root. We are maintaining the heap property at every stage. Note down example from board Complexity? For (k = empty_pos; k >1; k = k/2 ) If (Heap[k] < Heap[ k/2 ]) interchange(Heap[k], Heap[ k/2 ]) ;

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DeleteMin Delete the root. Compare the two children of the root Make the lesser of the two root. An empty spot is created. Bring the lesser of the two children of the empty spot to the empty spot. A new empty spot is created. Continue

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Note down example from the board. What is the complexity? Do we maintain the heap property in this procedure? Completeness is not preserved. Note down the example from the board. We can delete the last element in the heap, insert is at the empty spot at a leaf, and then move it upwards as necessary.

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pseudocode Delete root; k =1; Do { If A[k] is empty, break; If A[k] min(A[2k], A[2k + 1]), break; If A[2k] < A[2k + 1] j = 2k, else j = 2k+1; A[k] = A[j]; k = j; }Insert A[currsize] at A[k]; }

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Heap Sort You want to sort n real numbers. Insert them in a heap; Deletemin n times; Complexity O(h), h is the depth of the tree

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Increase Key Need to increase the value of an element. Increase the value of the element, Interchange it with the lesser of its children if it is greater than any of its children, Continue doing this till you reach a leaf

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Decrease Key Need to decrease the value of an element. Decrease the value of the element, Interchange it with its parent if it is less than its parent, Continue doing this till you reach the root

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Tighter Analysis of Complexity of Build Heap Suppose we have n elements. We want to build a heap of n elements. We present an O(n) algorithm to do so. Insert these elements in n positions of an array. Is an almost complete binary tree. But does not satisfy the heap order So we need to interchange these elements to obtain heap order

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Parents are at position n/2 to 1. If we percolate these down suitably, (interchange with lesser of the two children, if it is less than either of the two children, and so on), then we will get heap order. For (j = n/2; j--; j >0) Percolate Down (j);

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Percolate Down (j) { If (2j > n) break; If Heap[j] min(Heap[2j], Heap(2j+1), break; If (Heap[2j] < Heap[2j + 1]) { interchange(Heap[j], Heap[2j]); Percolate Down (2j);} else { interchange(Heap[j], Heap[2j+1]); Percolate Down (2j+1);} }

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Complexity Analysis Complexity of percolating down a single element is O(h) where h is the height of the element. Overall complexity is O(sum of the heights of all elements) We will show that sum of the heights of all elements is O(n). For an almost complete binary tree, There is 1 element at height h, 2 at height h-1 4 at height h-2…… 2 i at height h-i

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The number at height 0 is between 1 and 2 h However, these elements do not contribute to the sum of the heights. Thus sum of the heights is j=0 h-1 2 j (h-j) This is equal to 2 h+1 - 1 -(h+1) We have seen that the number of nodes n is greater than 2 h-1 + 2 and 2 h-1 + 2 is greater than (1/4) (2 h+1 - 2 –h) Thus sum of heights is less than 4n Thus sum of heights is O(n)

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