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2004_fall_exam1_sol

# 2004_fall_exam1_sol - P1 Solve the recurrence relation...

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P1. Solve the recurrence relation without using Master’s theorem: T(N) = 3T(N/2) + cN Ans.: Assume N = 2 k T(2 k ) = 3 T(2 k-1 ) + c 2 k = 3 2 T(2 k-2 ) + c ( 2 k + 3* 2 k-1 ) …. After k steps = 3 k T(1) + c ( 2 k + 3* 2 k-1 + 3 2 * 2 k-2 + … + 3 k-1 * 2) = 3 k d + c 2 k (1 + 3/2 + (3/2) 2 + (3/2) 3 + …. + (3/2) k-1 ) = 3 k d + c N [ (3/2) k –1] / [3/2 –1] = 3 k d + 2c N ( 3 k /N – 1) = 3 k (d + 2c) – 2cN Now, 3 k = (2 log 3 ) k = (2 k ) log 3 = N log 3 = N log 3 (2c + d) – 2cN = Theta (N log 3 ) P2. A and B are playing a guessing game where B first thinks up an integer X (positive, negative or zero, and could be of arbitrarily large magnitude) and A tries to guess it. In response to A ’s guess, B gives exactly one of the following three replies: a) Try a bigger number b) Try a smaller number or c) You got it!! Design an efficient algorithm to minimize the number of guesses A has to make. An example (not necessarily an efficient one) below: B thinks up the number 35 A’s guess B’s response 10 Try a bigger number 20 Try a bigger number 30 Try a bigger number 40 Try a smaller number 35 You got it

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