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Hashing and Randomized Algorithms
- 6.1. You are given unbiased coins, and perform the following process to generate all heads. Toss all coins independently at random onto a table. Each round consists of picking up all the tails-up coins and tossing them onto the table again. You repeat until all coins are heads.
- (a) What is the expected number of rounds performed by the process?
- (b) What is the expected number of coin tosses performed by the process?
- 6.2. Suppose we flip coins each of known bias, such that is the probability of the th coin being a head. Present an efficient algorithm to determine the exact probability of getting exactly heads given .
- 6.3. An inversion of a permutation is a pair of elements that are out of order.
- (a) Show that a permutation of items has at most inversions. Which permutation(s) have exactly n(n - 1)/2 inversions?
- (b) Let P be a permutation and be the reversal of this permutation. Show that and have a total of exactly inversions.
- (c) Use the previous result to argue that the expected number of inversions in a random permutation is .
- 6.4. A derangement is a permutation of such that no item is in its proper position, that is, for all . What is the probability that a random permutation is a derangement?
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