Assignment 13 -- Due Monday, December 7 (at the beginning of lab)
CS 4521 Fall Semester, 2009
25 Points

### Topics: Minimum Spanning Trees and Single Source Shortest Paths

The assignment consists of two parts. In the first part you will hand-trace the algorithms of Kruskal and Prim to find minimum spanning trees. In the second part, you will implement Dijkstra's Algorithm to solve the single-source shortest paths problem.

### Part I: Minimum Spanning Tree Exercises (10 points)

Consider the following undirected graph:
```             5
(B)----------(D)
|\         / | \
| \10   20/  |  \
|  \     /   |   \
|   \   /  12|   |
3|    \ /     |   |
|    (A)----(F)  |11
|        2    \  |
|             4\ |
|               \|
(C)--------------(E)
15

```
1. Trace the MST-Kruskal algorithm on this graph. Start by drawing the vertices only:
```     (B)          (D)

(A)    (F)

(C)              (E)

```
and show how the MST is grown by showing a snapshot of it after each edge is added.

2. Trace the MST-Prim algorithm on this graph starting with root C by showing the priority queue after each iteration of the while loop. Also show the key value for each node. The initial queue should look like:
```                        0 inf inf inf inf inf
Q: ( C  A   B   D   E   F )

```

### Part II: Implementation of Dijkstra's Algorithm (15 points)

For this part, implement Dijkstra's algorithm (page 595), and run it on two graphs. As discussed on page 599, the min-priority queue used in Dijkstra's algorithm may be implemented in four ways (each key value key[u]) will always be equal to the distance estimate d[u]):
• (15 points) an array implementation, in which the queue is maintained by the key[] array and an auxiliary array inQ[] of booleans that indicates whether vertex u is currently in the queue or not (inQ[u] would be true or false respectively). To initialize the queue (Line 3), for u = 1 to n insert d[u] into the priority queue. I changed the while-loop to a for-loop and iterated n times (though it is simple enough to keep count of the number of items in the queue and quitting the while-loop when this number drops to 0). Also, it is sufficient to iterate n-1 as indicated by Exercise 24.3-3 (whose short answer is "yes"). The initSingleSource() and relax() functions (and dijkstra() for that matter) can be member functions of the Graph class. To keep d[v] and key[v] "synchronized", Line 2 of relax() should involve two operations:
then (1) d[v] <- d[u] + w(u,v) and (2) DecreaseKey( Q, v, d[u] + w(u,v) )
(remember that this is a decrease-key operation). To implement extractMin(), just do a linear search for the location of the minimum key[] value that is still in inQ[]. Note that a vertex u is an array index and therefore a handle into the priority queue, and conversely an array index i of key[] or inQ[] is a handle to vertex i of the graph - moreover, neither of these handles ever change.
• (17 points) the binary heap or (19 points) binomial heap implementations. In these implementations, the vertices and corresponding heap elements must maintain handles to each other (though, since the vertices don't change position, the handle to a vertex (stored with its key) in the heap doesn't change once it is set initially; however the handles from the vertices into the heap will change -- they can be stored in an array handleToHeap[], so handleToHeap[u] points to the node in the heap with d's key value. These handles would be integer indexes or pointers for binary heaps or binomial heaps respectively.
• (25 points) the Fibonacci heap implementation. The main problem here is to implement a Fibonacci heap. You have to have the same kind of handles for the Fibonacci heap implementation as for the binomial heap implementation. But there is a subtle difference: once initialized, neither of the handles change, so this implementation is somewhat easier (once you have a Fibonacci heap).

Discussion:
Use numbers 1, 2, 3, 4, 5 instead of the letters s, t, x, y, z to identify the vertices. Also, the set S is only used to prove correctness of the algorithm -- so you don't have to include code for it. You can use a large number, say 1000 (I think 1 + the sum of the weights of all the edges is enough), instead of infinity in the initialization (I don't think there will be any arithmetic problems caused by using 1000 instead of INT_MAX or other fancy arithmetic). Also, you can use 0 for NIL, the intialization value for the pi field.

After initialization and at the end of each iteration of the while loop of Dijkstra's algorithm, your program should print out a list of all vertices, their d-values and their pi-values (it might also be useful for debugging to print out the priority queue). (Note: instead of the while loop, you can use a for-loop from 1 to n, though it would be easy to implement the empty() function for the priority queue - just keep track of the number of elements with "true" values in the inQ[] array.) Run this implementation on four different (G,w,source) combinations:

1. The graph and weights G,w of Figure 24.2 page 585, with source s ( = 1 ) - you may get one of the solutions shown in (b) or (c) of Figure 24.2, or something different: graph 24.2, source s. This is essentially Exercise 24.3-1 on page 600.
2. The graph and weights G,w of Figure 24.2 page 585, but with source z ( = 5 ): graph 24.2, source z.
3. The graph and weights G,w of Figure 24.6 page 596, with source s ( = 1 ) - you may very well get the solution of Figure 24.6, or something different: graph 24.6, source s.
4. The graph and weights G,w of Figure 24.6 page 596, but with source z ( = 5 ): graph 24.6, source z.
Also for each case, after the program run, draw the shortest-paths tree given by the predecessor graph. The data files have the number of vertices, n, and the source, source on the first line. Each subsequent line contains three values u v, and w, representing an edge, where the edge goes from vertex u to vertex v and has weight w.

Here is what the output from the first test should look like:

```Built graph, n = 5

Run Dijkstra with source = 1

Iteration 1, Distances, d[1] = 0, d[2] = 3, d[3] = 1000, d[4] = 5, d[5] = 1000

Iteration 2, Distances, d[1] = 0, d[2] = 3, d[3] = 9, d[4] = 5, d[5] = 1000

Iteration 3, Distances, d[1] = 0, d[2] = 3, d[3] = 9, d[4] = 5, d[5] = 11

Iteration 4, Distances, d[1] = 0, d[2] = 3, d[3] = 9, d[4] = 5, d[5] = 11

Iteration 5, Distances, d[1] = 0, d[2] = 3, d[3] = 9, d[4] = 5, d[5] = 11

Final distances and predecessors:

Distances, d[1] = 0, d[2] = 3, d[3] = 9, d[4] = 5, d[5] = 11

Predecessors, pi[1] = 0, pi[2] = 1, pi[3] = 2, pi[4] = 1, pi[5] = 4
```
This corresponds to Figure 24.2 (b) on page 585. It is possible that changing the order of the adjacency lists could produce the shortest paths tree of Figure 24.2 (c).
Figure 24.6 (page 596) shows how the third test run should proceed.

Helpful code: Here is a .h file for a new Graph class graph.h and skeleton code for the Graph class implementation graph.cpp (you implement parts associated with Dijkstra's algorithm). Note: the Graph class has been designed to work with both weighted and unweighted graphs - weighted in this case. Here is a .h file for the "array" priority queue class PQueue pqueue.h and code for the PQueue class implementation pqueue.cpp. Here is a "driver" program that tests Dijkstra's algorithm: driver.cpp. Here is a "Makefile" that puts it all together: Makefile. When you download these files, be sure to remove the ".txt" suffixes (by moving them to the same file name without the ".txt").

What to turn in:

• For Part 1, turn in your handwritten solutions to both parts.
• For Part 2, turn in: