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sol-quiz2

# sol-quiz2 - CMPT 307 Solutions to Midterm#2[3 1 True or...

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CMPT 307 Solutions to Midterm #2 November 13, 2008 1. True or False? (Below, Pr [ X ] stands for the probability of an event X .) [3] (a) For any random events A and B , Pr [ A & B ] = Pr [ A ] · Pr [ B | A ]. (b) For any random events A 1 , A 2 , . . . , A n , Pr [ A 1 A 2 ∨ · · · ∨ A n ] = n i =1 Pr [ A i ]. (c) For independent random events A 1 , . . . , A n , n i =1 Pr [ A i ] 1. Solution: (a) and (c) are True; (b) is False. 2. Explain very briefly why any comparison-based sorting algorithm will take time at [5] least Ω( n log n ) to sort n items. Solution: There are n ! possible orderings of n elements. A correct comparison-based algorithm must be able to produce any one of these orderings. Viewing the comparison- based algorithm as a decision tree, this means that the tree has n ! leaves. But then it must have at least log( n !) = Θ( n log n ) height (= running time of the algorithm). 1

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3. Dynamic Programming (a) Recall the string matching problem from HW 3: Given n, m N and binary [20] strings s, t 1 , . . . , t m , where n is the length of s , decide whether s can be expressed
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