CS 5114 Homework 2 (Spring 2008)

Describe your algorithms as clearly as possible. The style used in the book is fine, as long as your description is not ambiguous. Do not make any assumptions not stated in the problem. If you do make any assumptions, state them clearly, and explain why the assumption does not decrease the generality of your solution. Do not describe your algorithms only for a specific example you may have worked out. You must also provide a clear proof that your solution is correct (or a counter-example, where applicable).

1. (10 points) Solve exercise 2 in Chapter 4 (page 189) of your textbook.
2. (25 points) Solve exercise 13 in Chapter 4 (pages 194-195) of your textbook.
3. (30 points) Solve exercise 21 in Chapter 4 (page 200) of your textbook. Note that you cannot sort the edges, since your algorithm must run in O(n) time. (Hint: First solve exercise 2 in Chapter 3 (page 107) of your textbook. If you like, you can give your algorithm for this exercise some name, e.g., FindCycle, and use this algorithm as a sub-routine for solving exercise 21 in Chapter 4.)
4. (35 points) You are given a graph G(V, E) and a minimum spanning tree (V, T) for G. You now pick an edge e in E and change its cost from c to c'. Describe an algorithm to update T so that it is a minimum spanning tree for the graph with the new edge weight. Your algorithm must run in time linear in the number of nodes and edges in G. There are multiple cases to consider, depending on whether e is in T or not and whether c is larger or smaller than c'. You may find it useful to describe each case separately. You can assume that all edge costs are distinct, both before and after the change.