# upenn 502-04 - CIS 502 Spring 2004 1 Solutions to Final...

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CIS 502 - Spring 2004 (5/3/04) 1 Solutions to Final Exam May 3, 2004 Problem 1. You are given an unsorted array A of n positive integers (may not be distinct). Design an O ( n ) time algorithm to ﬁnd the smallest positive integer that does not occur in the array A . ( 12 points ) Solution: The smallest missing positive integer is at most n +1 since there are at most n distinct integers in A . Initialize a Boolean array M [1 .. ( n + 1)] to false. Scan A [1] through A [ n ], and for each A [ i ] n + 1, set M [ A [ i ]] to true to indicate that A [ i ] is not a candidate. Now output the least integer j such that M [ j ] is false. Since both A and M are of size O ( n ), the algorithm runs in O ( n ) time. Problem 2. Design a data structure that maintains a dynamic set S whose elements are distinct positive integers and supports the following operations: (a) Insert ( x ) and Delete ( x ); (b) Range ( x, y ): number of elements in S whose value lies strictly between x and y ; and (c) Sum ( x, y ): sum up the elements in S whose value lies strictly between x and y . Note that the elements x and y above are any two positive integers that may or may not be in the set S . You can assume that x y . All operations should run in O (log n ) worst-case time where n denotes the number of elements in the set. You can build upon any data structure described in the textbook or in the class. ( 18 points ) Solution: We maintain the set S by modifying the order statistics tree data structure to include in each node x an additional ﬁeld sum [ x ]. The ﬁeld contains the sum of the values of the elements in the subtree rooted at x (including x itself). We maintain the sum information similarly to size without aﬀecting the asymptotic running time for insert and delete operations. On insert, instead of adding 1 along the path traversed from the root down to the leaves, we add the value of the inserted element. When rotation is performed, we update the corresponding size ﬁelds using the information from the subtrees and the key values of the two nodes involved. Deletion is performed analogously. We deﬁne a function OS-Sum ( x ) that returns the sum of the elements x . Its implementation is analogous to OS-Rank .

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## This note was uploaded on 11/26/2009 for the course CIS 502 taught by Professor Naver during the Spring '09 term at National Tsing Hua University, China.

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upenn 502-04 - CIS 502 Spring 2004 1 Solutions to Final...

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