# sol3 - Introduction to Algorithms Solution Set 3 CS 482,...

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Unformatted text preview: Introduction to Algorithms Solution Set 3 CS 482, Spring 2008 (1) We describe an algorithm Test ( S ) whose input is a set S of bank cards and whose output is: a bank card x S such that more than half the elements of S are equivalent to x , if any such bank card exists; otherwise, the output is null . Let n = | S | . When n = 0, Test ( S ) outputs null . When n = 1, Test ( S ) outputs the unique element of S . Otherwise, Test ( S ) partitions S into two (arbitrary) subsets S 1 ,S 2 of size b n/ 2 c and d n/ 2 e . Let x 1 = Test ( S 1 ) ,x 2 = Test ( S 2 ). For each non- null element of the set { x 1 ,x 2 } , the algorithm takes this bank card and performs an equivalence test against all other bank cards. Let n 1 be the number of bank cards equivalent to x 1 (including x 1 itself), or n 1 = 0 if x 1 = null . Define n 2 similarly, but with x 2 in place of x 1 . If n 1 > n/ 2 then Test ( S ) outputs x 1 ; else if n 2 > n/ 2 then Test ( S ) outputs x 2 ; else Test ( S ) outputs null . Running time. Equivalence-testing x 1 against all other bank cards takes n- 1 comparisons or 0 comparisons, depending on whether x 1 = null , and similarly for x 2 . Thus the running time, f ( n ), of Test ( S ) satisfies f ( n ) f ( b n/ 2 c ) + f ( d n/ 2 e ) + 2( n- 1) . The solution of this recurrence satisfies f ( n ) = O ( n log n ) . Correctness. The proof of correctness is by induction on n , the base cases n = 0 , 1 being trivial. The algorithm never outputs a bank card unless it has directly compared that bank card to all others and determined that it is equivalent to at least b n/ 2 c of them, hence it never produces an incorrect non- null output. We only need to prove that if there is a set T S of bank cards which are all equivalent, such that | T | > n/ 2, then the algorithm outputs an element of T . To prove this, let m = | T | , m 1 = | T S 1 | , m 2 = | T S 2 | . If 2 m 1 | S 1 | and 2 m 2 | S 2 | then 2 m | S | contradicting our assumption that | T | > n/ 2. So at least one of the following inequalities holds: m 1 > | S 1 | / 2 or m 2 > | S 2 | / 2 . Assume without loss of generality that the first of these holds. Then Test ( S 1 ) outputs an element x 1 T , and Test ( S ) will compare x 1 with all other elements of S and discover that the set of elements equivalent to x 1 (i.e., the set T ) contains more than half the elements of S . Hence Test ( S ) will output an element of T , as desired. (2) We present two solutions. The first is a divide-and-conquer algorithm which resembles mergesort in that it divides the input into two equal-sized subproblems, solves both subproblems, then combines the results by carefully walking through a pair of ordered lists. The second is a simpler algorithm that sorts the lines and processes them in order of increasing slope, maintaining a stack whose contents are all the lines currently visible. Solution 1: Let = { L 1 ,...,L n } be any collection of nonvertical lines. Assume without loss ofbe any collection of nonvertical lines....
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## This note was uploaded on 10/02/2008 for the course CS 482 taught by Professor Kleinberg during the Spring '08 term at Cornell University (Engineering School).

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sol3 - Introduction to Algorithms Solution Set 3 CS 482,...

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