CSCI 3104 Problem Set 2


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1. (20 pts total) Solve the following recurrence relations using any of the following methods: unrolling, tail recursion, recurrence tree (include tree diagram), or expansion.
Each each case, show your work.
(a) T(n) = T(n − 2) + C n if n > 1, and T(n) = C otherwise
(b) T(n) = 3T(n − 1) + 1 if n > 1, and T(1) = 3
(c) T(n) = T(n − 1) + 3n
if n > 1, and T(1) = 3
(d) T(n) = T(n
) + 1 if n > 2 , and T(n) = 0 otherwise
2. (10 pts) Consider the following function:
def foo(n) {
if (n > 1) {
print( ’’hello’’ )
In terms of the input n, determine how many times is “hello” printed. Write down a
recurrence and solve using the Master method.
3. (30 pts) Professor McGonagall asks you to help her with some arrays that are raludominular. A raludominular array has the property that the subarray A[1..i] has the
property that A[j] > A[j + 1] for 1 ≤ j < i, and the subarray A[i..n] has the property that A[j] < A[j + 1] for i ≤ j < n. Using her wand, McGonagall writes the following raludominular array on the board A = [7, 6, 4, −1, −2, −9, −5, −3, 10, 13], as an example. (a) Write a recursive algorithm that takes asymptotically sub-linear time to find the minimum element of A. 1 (b) Prove that your algorithm is correct. (Hint: prove that your algorithm’s correctness follows from the correctness of another correct algorithm we already know.) (c) Now consider the multi-raludominular generalization, in which the array contains k local minima, i.e., it contains k subarrays, each of which is itself a raludominular array. Let k = 2 and prove that your algorithm can fail on such an input. (d) Suppose that k = 2 and we can guarantee that neither local minimum is closer than n/3 positions to the middle of the array, and that the “joining point” of the two singly-raludominular subarrays lays in the middle third of the array. Now write an algorithm that returns the minimum element of A in sublinear time. Prove that your algorithm is correct, give a recurrence relation for its running time, and solve for its asymptotic behavior. 4. (15 pts extra credit) Asymptotic relations like O, Ω, and Θ represent relationships between functions, and these relationships are transitive. That is, if some f(n) = Ω(g(n)), and g(n) = Ω(h(n)), then it is also true that f(n) = Ω(h(n)). This means that we can sort functions by their asymptotic growth.1 Sort the following functions by order of asymptotic growth such that the final arrangement of functions g1, g2, . . . , g12 satisfies the ordering constraint g1 = Ω(g2), g2 = Ω(g3), . . . , g11 = Ω(g12). n n 1.5 8 lg n 4 lg∗ n n! (lg n)! 5 4 n n 1/ lg n n lg n lg(n!) e n 42 Give the final sorted list and identify which pair(s) functions f(n), g(n), if any, are in the same equivalence class, i.e., f(n) = Θ(g(n)). 1The notion of sorting is entirely general: so long as you can define a pairwise comparison operator for a set of objects S that is transitive, then you can sort the things in S. For instance, for strings, we use a comparison based on lexical ordering to sort them. Furthermore, we can use any sorting algorithm to sort S, by simply changing the comparison operators >, <, etc. to have a meaning appropriate for S. For instance, using Ω, O, and Θ, you could apply QuickSort or MergeSort to the functions here to obtain the sorted list.