SOLUTIONS for practice final for EECS 380, 2001
Profs Markov and Brehob

Available in Postscript and PDF
Total pages: 5
Exam duration: 1hr 50min.
Write your name and uniqname on every sheet, including the cover.

Maximum score: 100 points + 15 extra. Extra credit points do not affect the curve.
To be eligible for extra credit, you need to earn at least 70 regular points.

All complexity estimates are for runtime (not for memory), unless specified otherwise.

  1. 30 points. Algorithmic Complexity
  2. Each line in the table corresponds to an algorithm or an algorithmic problem. Write P for problems and A for algorithms. A problem gives input and output, but an algorithm additionally entails a particular method of achieving this output. Fancy data structures (e.g., heaps, BSTs and hash-tables) often imply specific algorithms. Simple containers (e.g., arrays and linked lists) are typically used to store input or output and may restrict possible algorithms.

    For each algorithm, write its Theta-complexities.
    For each problem, write Theta-complexities of a best possible algorithm that solves the problem.
    There can be multiple correct answers, especially, if there is a trade-off between average-case and worst-case performance.
    No explanation necessary.

    You can assume that operator< and operator== for values stored in containers run in O(1) time.
    You cannot make any additional assumptions about algorithms/problems unless instructed by Prof. Brehob or Prof. Markov.

    Each line is worth 2 points. Each wrong or missing answer on a line costs -1 point.
    Minimum per line = 0 points.
    Algorithm or Problem:   ? Best-case Theta()  Avg-case Theta()  Worst-case Theta() 
    1.  Find a given value in an unsorted N-by-N matrix.  P 1 N2 N2
    2.  Binary search over N elements  A log N log N
    3.  Find the largest element in an unsorted array with N elements   P   N   N   N 
    4.  Print all values appearing at least twice in a sorted stack of size N  P   N   N   N 
    5.  Insert a new element into a sorted singly-linked list with N elements 
    so that the list remains sorted 
     P   1   N   N 
    6.  Given two unsorted arrays of N and N/10 elements, say whether they have at least one common element   P   1   N log N   N log N 
    7.  Shaker sort of a doubly-linked list with N elements, using "early termination".   A   N   N2   N2 
    8.  Duplicate a queue of N elements   P   N   N   N 
    9.  One invocation of the partition() function used in the quicksort algorithm. Assume in-place partitioning of a complete array with N elements using a given pivot   P/A   N   N   N 
    10.  Given a pointer to an element in a singly-linked list with N elements, remove that element from the list   P   1   1*   N** 
    11.  Sort N 8-bit characters stored in an array.    P   N   N   N 
    12.  Remove the middle element from an unsorted array of N elements   P   1   1   1 
    13.  Compute N! for a given N using a straightforward recursive algorithm   A   N   N   N 
    14.  Find the combination of N decimal digits that opens a bank safe. The safe opens when you enter the right combination, and you can try as many combinations as you wish. No other feedback is available   P   1   10N   10N 
    15.  Print all diagonal values of a given N-by-N matrix   P   N   N   N 
    * Suppose the pointer points to A. Copy the successor B into A and remove old copy of B.
    ** The worst-case happens when A is the last element. (This can be prevented with a sentinel)

  3. 10 points. STL
  4. Fill in the blanks

    1. "STL" stands for ____Standard Template Library______
    2. A range can be defined by two __iterators______
    3. STL's sort() and binary_search() functions take an optional _comparison_ function-object
    4. One can use class __map__ from STL as an implementation of Abstract Symbol Table.
    5. Iterators of linked list classes in STL do not allow __random_ access.

  5. 20 points. Fancy containers (heaps, generic trees, search trees, hash-tables, etc)
    1. 10 points. Follow instructions from Question 1.

      Algorithm or Problem:  Best-case Theta()  Avg-case Theta()  Worst-case Theta() 
      1.  Print all values stored at nodes of a given tree with N nodes   N   N   N 
      2.  Convert a binary heap of N elements into a sorted array    don't bother   NlogN   NlogN 
      3.  Test whether a given array with N values is in a binary-heap order   1   1 or N   N 
      4.  One search in a BST of N elements. Assume that the tree is perfectly balanced and the search results in a miss   log N   log N   log N 
      5.  One successful look-up in a hash table with N elements and load ratio* 1.0. The hash-table uses separate chaining with singly-linked lists. Assume that hash-function can be computed in O(1) time.
      Note: elements contained in the hash-table may be poorly dispersed
       1   1   N 

    2. * The load ratio of a hash-table with N elements and M buckets is N/M.

    3. 5 points. Consider struct Key { char p1, p2, p3 };
      and the following hash-functions (modulo hash-table size).
      1. unsigned f1(struct Key& s) { return s.p1+5*s.p2; }
      2. unsigned f2(struct Key& s) { return 10*s.p1+100*s.p2+1000*s.p3; }
      3. unsigned f3(struct Key& s) { return 11*s.p1+101*s.p2+1001*s.p3; }
      Assume a hash-table of size 1250 with linear probing.
      Mark each hash-function as good or bad. Use space below to explain.

      • f1() does not depend on p3 therefore keys that only differ at p3 will not be dispersed. BAD
      • All values of f2() are divisible by 10. Since the table size is also divisible by 10, at most 10% of the hash buckets can be used w/o hash collisions. BAD
      • f3() depends on all fields and is a linear function whose coefficients are relatively prime with the table size. GOOD

    4. 5 points. Fill in the blanks.
      Markov section only
      In BSTs, _left_ and _right_ rotations have time complexity Theta(_1_). They are explicitly used in _root__ insertion and _partitioning_ algorithms. Two BSTs can be joined using a _recursive_ algorithm, which applies _root_ _insertion_ to one of the trees. The worst-case complexity of such a join algorithm is Theta(_N2_), but the best case can be faster when __a pivot exists such that all values in the first tree are smaller and all values in the second tree are larger than the pivot__.

      Brehob section only
      Each node in a 2-3-4 tree has _1__, _2__ or _3__ keys in it. _red-black_ trees are an implemention of 2-3-4 trees. Insertion into a 2-3-4 tree has worst-case complexity Theta(_logN_) and search has worst-case complexity Theta(_logN_).

  6. 20 points. Algorithm design: Recursion / Divide and Conquer / Dynamic Programming
  7. Implement the following C++ function

      void makeBalancedBST(unsigned *begin, unsigned numElem);

    which takes an unsorted array and makes a balanced BST out of it, stored left to right so that children of element k be 2*k and 2*k+1. You must achieve worst-case complexity O(numElem log2(numElem)) and explain how you did it. 15 points for the case when numElem is a power of two minus one (say, 3, 7 or 15), 5 additional points for the general case. Use a separate page.

    Solution: a a complete working program for the general case is provided. Another complete program sent in later by Nick Schrock shows a much shorter solution. It may run slower, but is great if you need to write a piece of code quickly on a final exam.

  8. 20 points. Questions related to HWKs and Projects
    1. 5 points. Provide a dictionary produced by the Huffman algorithm applied to this input: AAABAABCCDCC. No explanation necessary.

      Explanation: Frequencies: A(5), C(4), B(2) and D(1). Huffman algorithm: first merge the least frequent letters B and D (cumulative frequency is 3). Then merge the least frequent letters/subtrees: BD and C (cumulative frequency 7). Then merge the resulting subtree with A. One of possible ways to assign bits to the edges of the tree gives the following prefix-free dictionary.

      Answer: A: 0, C: 10, B:110, D:111
      (alternative correct answers are possible)

    2. 5 points. Heapify the digits of your student ID. Start with the digits in the original order and show the process step by step.

      Solution: for this problem one can use the linear-time make_heap algorithm or call push_heap N times. The latter may be easier to remember, but requires more work.

      Linear-time make_heap on (1 2 3 4 5 6 7):
      (1 2 3 4 5 6 7)   (1 2 7 4 5 6 3)   (1 5 7 4 2 6 3)
        (7 5 1 4 2 6 3)   (7 5 6 4 2 1 3)

    3. 10 points. You are given a function that takes N planar points and returns all points on the boundary of the convex hull listed clockwise. Provide an algorithm (in pseudocode or valid C++) that sorts N doubles using that function and spends O(N) time outside that function.

      • Find the smallest and the largest values (one linear-time pass).
      • Scale all original numbers by subtracting min and dividing by (max-min) (one linear-time pass).
        // The relative order is preserved, but all numbers are now between 0 and 1.
      • For every number alpha, compute the point on the unit circle whose polar angle is alpha. The exact formulae for coordinates are x=cos(alpha), y=sin(alpha) (one linear-time pass).
        // Note that pi=3.1415926... and pi/2>1. // Therefore, the points will not "wrap up" around the circle.
      • Run the convex hull algorithm.
        // Note that that all those points will be on the convex hull.
        // Additionally, the convex hull algorithm orders the points clockwise.
      • Read off the points in the clockwise order and apply inverse transformations: find alpha using the atan2() function or otherwise, then multiply by (max-min) and add min (one linear-time pass).

  9. Extra credit: 15 points. ``Comments not available''.
  10. In this question you are given a printout of a C++ function, with coke spilled over the comments (=> you can't read the comments). You need to explain what the function does, illustrate by several representative examples, give worst-case/best-case Theta() for runtime and substantiate these complexity estimates.

           int L2(const char * A, const char * B)
               int m=strlen(A), n=strlen(B), i, j;
               int L[m+1][n+1]; // g++ extension to C++
               for (i = m; i >= 0; i--)
                 for (j = n; j >= 0; j--)
                    if (A[i] == '\0' || B[j] == '\0') { L[i][j] = 0; }
                    else if (A[i] == B[j]) L[i][j] = 1 + L[i+1][j+1];
                    else L[i][j] = max(L[i+1][j], L[i][j+1]);
               return j;
    Source code courtesy of Prof. David Eppstein.

    Solution (idea only): This program computes the length of the longest common subsequence of two sequences. Its complexity is Theta(mn) where m and n are the lengths of the two sequences. The asymptotic runtime is the same in all cases due to the two nested loops w/o break instructions inside.