final-s2009-sol

# final-s2009-sol - Introduction to Algorithms Massachusetts...

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Introduction to Algorithms May 21, 2009 Massachusetts Institute of Technology 6.006 Spring 2009 Professors Sivan Toledo and Alan Edelman Final Exam Solutions Final Exam Solutions Problem 1. True or False [24 points] (8 parts) For each of the following questions, circle either T (True) or F (False). Explain your choice. (No credit if no explanation given.) (a) T F There exists an algorithm to build a binary search tree from an unsorted list in O ( n ) time. Explain: Solution: False. Because an in-order walk can create a sorted list from a BST in O ( n ) time, the existence of such an algorithm would violate the Ω( n lg n ) lower bound on comparison-based sorts. (b) T F There exists an algorithm to build a binary heap from an unsorted list in O ( n ) time. Explain: Solution: True. The standard BUILD - HEAP algorithm runs in O ( n ) time. (c) T F To solve the SSSP problem for a graph with no negative-weight edges, it is nec- essary that some edge be relaxed at least twice. Explain: Solution: False. Dijkstra’s algorithm solves the SSSP problem relaxing each edge at most once. (d) T F On a connected, directed graph with only positive edge weights, Bellman-Ford runs asymptotically as fast as Dijkstra. Explain: Solution: False. Bellman-Ford requires Θ( V E ) , regardless of the edge weights. Dijkstra runs in Θ( E + V lg V ) . Because the graph is connected, E = Ω( V ) , so Θ( V E ) = Ω( V 2 ) , which is clearly worse than Dijkstra.

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6.006 Final Exam Solutions Name 2 (e) T F A Givens rotation requires O (1) time. Explain: Solution: False. It may take O (1) time if the matrix is sparse, but in general a Givens rotation requires O ( n ) for a matrix with n columns. (f) T F In the worst case, merge sort runs in O ( n 2 ) time. Explain: Solution: True. Merge sort runs in O ( n lg n ) time which is O ( n 2 ) . (g) T F There exists a stable implementation of merge sort. Explain: Solution: True. If the items in the two lists being merged are equal, choose the item in the “left” half (the half that came first in the original array). (h) T F An AVL tree T contains n integers, all distinct. For a given integer k , there exists a Θ(lg n ) algorithm to find the element x in T such that | k - x | is minimized. Explain: Solution: True. I NSERT k , then find the P REDECESSOR and S UCCESSOR of k . Return the one whose difference with k is smaller. All three methods take Θ(lg n ) time.
6.006 Final Exam Solutions Name 3 Problem 2. Short Answer [32 points] (4 parts) (a) The eccentricity ǫ ( u ) of a vertex u in a connected, undirected, unweighted graph G is the maximum distance from u to any other vertex in the graph. That is, if δ ( u, v ) is the shortest path from u to v , then ǫ ( u ) = max v V δ ( u, v ) .

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## This note was uploaded on 01/20/2012 for the course CS 6.006 taught by Professor Erikdemaine during the Fall '08 term at MIT.

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final-s2009-sol - Introduction to Algorithms Massachusetts...

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