CMPSCI 311: Theory of Algorithms

David Mix Barrington

Fall, 2003

Solutions to Practice Final Exam

Exam posted Wednesday 10 December 2003

Solutions to be posted Friday 12 December 2003

Actual final exam (120 min) is Tuesday 16 December 2003

Questions in black, answers in blue.

Directions: There are 150 total points. Each of the first four 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) Let A, B, and C be decision problems and let "≤" denote poly-time reducibility. If A ≤ B and B≤ C, then A ≤ C must be true.

  TRUE. There is a function f from A-instances to B-instances such that x is in A iff f(x) is in B. There is a function g from B-instances to C-instances such that y is in B iff g(y) is in C. So x is in A iff g(f(x)) is in C. The composition of g and f is the poly-time reduction. It is poly-time because g runs in time polynomial in the length of f(x), which is polynomial in the length of x.

• Question 2 (10): (true/false with justification) Let f and g be functions. If f is in the class o(g), then f+g is in the class Θ(f).

  FALSE. Not only is it not always true, it's always false, since f+g is in Θ(g), and o(g) has no functions in common with Θ(g).

• Question 3 (10): (true/false with justification) Let G be a directed graph given by its adjacency matrix. We say that a vertex v in G is special if (u,v) is an edge for every vertex u. Determining whether a given vertex v is special requires Θ(n) time.

  TRUE. Certainly you can do it in O(n) time by checking for an edge (u,v) for every u -- there are n-1 u's taking O(1) time apiece. And you need to look at every u in the worst case, which is when the first n-2 u's you look at have an edge (u,v) and so the answer depends on the last one. So the time for the problem is in Ω(n) as well, and thus in Θ(n).

• Question 4 (10): (true/false with justification) If A and B are both decision problems in the class NP, then A ≤ B and B ≤ A cannot both be true. (Again, "≤" denotes poly-time reducibility.)

  FALSE. Both reducibilities are true if A and B are both NP-complete problems, since any NP problem reduces to any NP-complete problem. The existence of NP-complete languages does not depend on any unproven assumptions like P ≠ NP.

• Question 5 (30): Consider n items stored in a min-heap. The items may only be moved and compared to one another. Moves and comparisons take O(1) time each.

  • (a,10) Explain how to create, from this heap, an array that has all n items in sorted order, using O(n log n) time. Justify your time bound carefully.

    I perform the following operation n times: remove the minimum element of the heap, place it in the next entry of my array, and restore the heapness of the heap. The first two take O(1) each, the last takes O(n) in the worst case because we replace the top of the heap with the last element, which may take O(log n) moves downward to get to its correct place. The total time is thus n times O(log n) or O(n log n).

  • (b,20) Explain why such a sorted array cannot be created from the heap in o(n log n) time.

    Given an unsorted array, we can create a heap in O(n) time using the algorithm in the book where we place the elements bottom-up and row by row, adjusting each subheap when we place its root. If we could go from the heap to the sorted array in o(n log n) time, combining this with the O(n) heapification we would have a general sorting algorithm using o(n log n) time, which is impossible using only comparisons and moves by the sorting lower bound.

• Question 6 (50): Suppose we are given an n by n matrix of booleans called A, denoting a binary relation, and we want to compute the n by n matrix of booleans B that denotes the reflexive transitive closure of this relation. Let m be the number of entries of the matrix that are "true".

  • (a,10) Carefully describe a reinterpretation of this problem in terms of a directed graph -- what information about the graph is given and what information is asked for?

    We let the matrix A be the adjacency matrix of a directed graph G. To get the graph corresponding to B, the reflexive transitive closure of A, we add self loops at each vertex and add an edge from u to v whenever a path from u to v exists in G.

  • (b,10) Explain how to find B by matrix powering. What is the running time of this method, assuming the ordinary and not the Solovay-Strassen method is used to multiply matrices?

    B is equal to (A+I)^n-1, where I is the identity matrix and the matrix multiplication is boolean, with AND for multiplication and OR for addition. We need log n matrix multiplications if we use repeated squaring. Since each matrix multiplication takes Θ(n³) time, the total running time is Θ(n³ log n).

  • (c,15) Explain how dynamic programming can be used to improve this time asymptotically (by Warshall's algorithm). What is the new running time?

    Warshall's algorithm computes B in time O(n³). It successively computes matrices A+I=C₀,...,B=C_n, where the (i,j) entry of C_k is a boolean telling whether there is a path from i to j using only intermediate vertices numbered at most k. Getting C_i+1 from C_i takes O(n²) time, so the total time is O(n³.

  • (d,15) Suppose m is in the class o(n²). Explain how to determine B asymptotically faster than by Warshall's algorithm. State and justify the new running time in terms of n and/or m.

    For each vertex i, run a depth-first search of the graph starting from i and set the (i,j) entry of B to be true iff j is found during this search. Each DFS takes O(n+e) time, so we have total time O(n²+ne). If e is in the class o(n²), this time is in the class o(n³).

• Question 7 (30): Suppose we are given an array of n integer values and we want to decide whether they are an arithmetic progression when viewed as a set. An arithmetic progression is defined to be a set of the form {a,a+b,a+2b,...,a+(n-1)b} for some integers a and b. Thus, our decision problem is to determine whether when sorted, the array is in this form.

  • (a,15) Show that this decision problem is in the class NP by describing an NP-procedure that can possibly return "true" if and only if the actual answer is "true". Your evaluation phase should take only O(n) time.

    The guess phase guesses an ordering of the vertices and guesses the number b. The evaluation phase verifies that the each element in the ordering is exactly b greater than the previous one, and that there are n elements in the ordering. If the set is an arithmetic progression, the correct ordering and b might be guessed, and if there is an ordering and a b satisfying the evaluation phase, we have an arithmetic progression. We can represent the ordering as an array of indices into the first array, that is, an array of numbers from 0 through n-1. If the guess phase does not guess a permutation, the evaluation phase will fail, so we need not check element distinctness for the ordering array.

  • (b,15) Explain how to solve this decision problem in O(n) time (with an ordinary algorithm, not an NP-procedure).

    Make one pass through the input to find the minimum element and call this a. Make another pass to find the maximum element z. If z-a is not divisible by n-1, return false, else set b to be (z-a)/(n-1). Now Make a new boolean array F of size n. For each element x of the array, if (x-a)%b is not zero, return false, else set F[(x-a)/b] to true. If you finish this task without returning false, check whether F is now all true and return true iff it is. Each of these passes through an array of size n takes O(n) time and we needed only O(1) of them for total time O(n).

    This looks like the element distinctness problem, which requires Ω(n log n) time if you may use only comparisons. But the items for which we must check distinctness are integers in a small range, and we can use a bitvector as above.

  Last modified 12 December 2003