CMPSCI 311: Theory of Algorithms

David Mix Barrington

Fall, 2003

Practice Exam #1

Posted Saturday 27 September 2003

Solutions to be posted Wednesday 1 October 2003

Actual (in-class) exam is Friday 3 October 2003

Directions: Each of the first two questions is a statement that may be true or false. Please state whether it is true or false -- you will get five points for a correct boolean answer and there is no penalty for a wrong guess. Then justify your answer -- the remaining points will depend on the quality and validity of your justification.

• Question 1 (10): (true/false with justification) If f(n) and g(n) are both increasing functions, each of which is in Ω(h(n)), then the function f(n) +g(n) is also in Ω(h(n)).

• Question 2 (10): (true/false with justification) Let T(n) be a nondecreasing function such that wheneve n = 2^k, T(n) = 2ⁿ. (Thus for these n's, T(n) = 2^2k.) Then T(n) is in Θ(2ⁿ).

• Question 3 (20): Solve the following recurrence exactly and prove by induction that your answer is correct:

  T(n) = 2T(n-1) - T(n-2) for n ≥ 2; T(0) = 3; T(1) = 3

• Question 4 (40): Let A be an array of n Comparables, no two of them equal. The median of A is defined to be the element that is greater than exactly n/2 other elements. (Here "n/2" means the floor of n/2 as in Java integer division.)

  • (a,20) Describe an algorithm to determine the median by brute force, according to the definition above. This algorithm should have worst-case running time of Θ(n²) -- if it is faster than that, then it is not "brute-force" but you may use it for part (c). Pseudocode is fine if it is perfectly clear what your algorithm does.

  • (b,10) What is the average-case running time of your algorithm, assuming that the median is equally likely to be in any position of A? Justify your answer.

  • (c,10) Briefly indicate a method to find the median that is asymptotically faster than Θ(n²) (that is, its running time is not Ω(n²)). Justify your answer.

• Question 5 (20): Consider a version of Quicksort that uses the brute-force median algorithm of Question 4 to find its pivot. Write a recurrence for the running time of this algorithm and solve it (in Θ terms) using the Master Theorem.

Last modified 27 September 2003