Question 1

• Part 1: The worst case running time of Quicksort is O(n²) while Merge Sort is always O(n log(n)). A is faster.
• Part 2: DFS is O(n). Network Flow is much, slower, as each augmenting path has to take at least O(n). A is faster.
• Part 3: Unweighted shortest path is a simple BFS: O(n). DFS is O(n). They are the same.
• Part 4: Heap Sort is O(n log(n)). The worst (and average) case of Insertion sort is O(n²). A is faster.
• Part 5: They are the same. Even though merge sort runs faster, their big-O complexity is the same.
• Part 6: Well, I had assumed that you knew the distribution of the data (or that the range of data is small) so that bucket sort would be linear. However, if you don't know the distribution, bucket sort defaults to insertion sort. That's a difficult one to analyze, so I gave full credit for any answer.
• Part 7: Union operations are O(1). Find operations are O(α(n)), where α is the inverse Ackerman function. So even though the inverse Ackerman of eight hundred bazillion is less than ten, A is faster.
• Part 8: Prim and Dijkstra's algorithms have the exact same mechanics -- they are both breadth-first searches that grow a tree with an edge map. They are the same.
• Part 9: Kruskal's algorithm has to sort the edges, so it is at least O(n log(n)). Each Union operation is O(1), so n of them are O(n). B is faster.
• Part A: Heap Sort is O(n log(n)). Cycle detection is DFS, which is O(n): B is faster.
• Part B: From part three -- A is O(n). Connected component determination is another DFS-based algorithm. They are the same.
• Part C: Matching on a bipartite graph is a network flow algorithm, which is way more expensive than the O(n α(n)) operations required for n Find operations.
• Part D: Both of these are linear: They are the same. Again, if you assumed that bucket sort didn't know the distribution, then B would be faster, so I gave full credit for "B is faster".
• Part E: Network flow is more expensive: B is faster. I gave .5 points for saying "A is faster", since you may have considered that "Edmonds-Karp" meant finding the path and not the whole network flow algorithm.
• Part F: This is O(n), because it is ten iterations through an n element array. Prim's algorithm is O(n log(n)). A is faster.

Grading

How Y'all Did

Question 2

Grading

Question 3

For Kruskal's algorithm, you first sort the edges, then use Union find to add edges to the MST. If the edges connect nodes in different sets, then you add the edge to the tree and perform Union on the two nodes' sets. Here are the states. The answer was a slow-pitch -- the first 8 edges in their sorted order: AD, GH, HF, GE, FI, BC, CF, DG.

Grading

Question 4

int Graph::Shortest_Path(Node *a, Node *b)
{
  int i;
  list <Node *> bfsq;
  list <Node *>::iterator bit;
  Node *n;

  for (i = 0; i < nodes.size(); i++) nodes[i]->tmp = -1;
  a->tmp = 0;
  bfsq.push_back(a);

  while (!bfsq.empty()) {
    bit = bfsq.begin();
    n = *bit;
    bfsq.erase(bit);
    if (n == b) return n->tmp;
    for (i = 0; i < n->edges.size(); i++) {
      if (n->edges[i]->tmp == -1) {
        n->edges[i]->tmp = n->tmp+1;
        bfsq.push_back(n->edges[i]);
      }
    }
  }
  return -1;
}

To test this, I hacked up a little main() which first creates a ten-node graph where node i has an edge to node i+1. It then prints out the path distance from node 0 to node 9 (which should be 9) and the path from node 9 to node 0 (which should be -1).

Then, for each node i I add edges to every node j such that j < i, and print out the same path lengths. Node 0 to node 9 should still have a path length of 9. Node 9 to node 0 should have a path length of 1. The code is in bfs.cpp

main()
{
  Graph g;
  Node *n;
  int i, j;

  for (i = 0; i < 10; i++) {
    n = new Node;
    g.nodes.push_back(n);
  }

  for (i = 0; i < 9; i++) {
    g.nodes[i]->edges.push_back(g.nodes[i+1]);
  }

  cout << g.Shortest_Path(g.nodes[0], g.nodes[9]) << endl;
  cout << g.Shortest_Path(g.nodes[9], g.nodes[0]) << endl;

  for (i = 0; i < 10; i++) {
    for (j = 0; j < i; j++) {
      g.nodes[i]->edges.push_back(g.nodes[j]);
    }
  }

  cout << g.Shortest_Path(g.nodes[0], g.nodes[9]) << endl;
  cout << g.Shortest_Path(g.nodes[9], g.nodes[0]) << endl;
}

UNIX> bfs
9
-1
9
1
UNIX> 

Grading

• Excellent/Good: A correct structure based on a BFSQ: 12 points
• Recursive structure that gave a correct answer: 9 points
• Used a BFSQ, but not well: 6 points
• No BFSQ/Just looping through a, incorrect recursive structure: 4 points.

• Needs a separate BFSQ -- you used nodes instead: 3 points.
• Lots of confused stuff going on: 3 points -- this was if you had too much nonsensical code.
• Length Calculated Incorrectly: 3 points -- this wasn't minor -- if you had this deduction, you were not using the tmp field to calculate the shortest path, which means that you weren't using BFS correctly.
• Too much incorrect syntax: 2 points -- similar to "confused stuff" above, but not as major.
• Returns 0 when there's no path: 1 point.
• Don't set tmp for any nodes: 1 points

Question 5

• The maximum flow is 60. Here is a flow graph:

• From the flow graph, it is easy to see that the minimum cut is composed of the edges BE and DE.
• Here are the answers for Greedy DFS:

• Here are the answers for Modified Dijkstra:

• Here are the answers for Edmonds-Karp:

Grading

Question 6

double CalcX::X(int x, int y)
{
  int i;
  double retval, tmp;

  // Set up the cache if need be.

  if (cache.size() < x+1) {
    cache.resize(x+1);
    for (i = 0; i <= x; i++) {
      if (cache[i].size() < y+1) cache[i].resize(y+1, -1);
    }
  }

  // Return the value from the cache if it is there.

  if (cache[x][y] != -1) return cache[x][y];

  // Otherwise, do the recursive calculation

  retval = 0;
  if (x == 0 && y == 0) retval = 1;
  if (x > 0) {
    tmp = F1(x-1, y);
    if (tmp > retval) retval = tmp;
  }
  if (y > 0) {
    tmp = F2(x, y-1);
    if (tmp > retval) retval = tmp;
  }

  // Put it into the cache and return

  cache[x][y] = retval;
  return retval;
}

Grading

• Excellent/Good: 12 points
• Zero: 0 points

• Dynamic program without a cache: 4 points. This was major, because setting up the cache was an important part of the program.
• Uses a for loop to build the cache without recursion: 3 points. This was also major -- yes, you can structure this as a doubly-nested for loop, but the question said to make it recursive with a cache.
• Cache not used as a vector, but some weird associative array: 3 points.
• Weird recursion -- set the cache for x,y-1 and y,x-1, never for x,y.: 2 points. These worked correctly, but will perform a factor of two worse than doing it the right way (as in the answer above).
• Both loops and recursion -- performance is going to be terrible: 2 points.
• Never-Check-Cache: 2 points. This i when you never use the cache.
• Don't deal with X(0,y) or X(y,0) correctly. : 2 points. A lot of you got this deduction, and many of you got it because you returned -1 when x or y was less than zero. Then you made a recursive call and didn't check that you were calling with a negative value, and certainly having it return -1 isn't going to eliminate that case. If you just said "negative infinity", I counted it as ok. I said that I don't care what you return when x or y is negative, which is true -- I do care that you get the correct values when x or y is equal to zero, which many of you failed to do.
• Don't Set Cache On Return: 2 points. This is when you forgot to set the value in the cache when you returned from the procedure.
• Confused resizing of the cache: 2 points.
• Use Cache Before Resize : 1 point.
• The cache is too small: only resize to x and y: 1 point. The cache needs to be resized to x+1 and y+1.
• Forgot about x == 0 & y == 0 or got it wrong: 1 point.
• Cache never initialized: 1 point.