CS 112b
Spring 2001
Ileana Streinu

Wrap-up, general trees

• general (non-binary) trees.
• Getting started on the final project: design a general Tree class, with a functionality able to model a Unix-like file system. I.e. allow functions to insert nodes ("mkdir" or create a file), navigate (cd, including cd ..), print the information in a node (ls) or in the whole tree (ls -r).

Graphs and graph traversal

• Graphs. Trees as special cases of graphs.
• Adjacency list representation. How the representation relates (or not) to the representation of (general, not binary) trees. Directed and undirected graphs.
• Implementation of a (Directed) Graph data structure: basic ideas. The complete class and struct definitions are part of next homework.
• Homework 10
• User interface. See for example the executable file testGraph in the directory ~112a/handout/Graph/. To go there, log on your 112a-xx account, then do cd ~112a/handout/Graph. To run testGraph, type testGraph at the Unix prompt. This is the minimal interface I expect.
• Discussion of the data structure.

Next week, continuation.
Graph Traversal, Depth First Search and Connectivity

• Directed graphs versus undirected graphs. Examples.
• Paths in graphs (directed and undirected). Examples.
• Connected versus not connected graphs. Examples.
• Connected digraphs for which there exists no path from a node A to a node B. Examples.
• Graph traversal algorithm: starts at a node v and "visits" all the nodes that can be reached by directed paths from v.
• Depth-first search (DFS) from node v: from each node, follow a path as far as you can, while visiting only vertices that you have not previously seen. When you reach a previously visited vertex, back up to where you came from and visit the next child of the current node, if any exists. Otherwise back up and repeat this procedure. Stop when you are at v and have backed up from its last child.
• Example of how DFS works.
• Implementation of DFS in pseudo code. Recursion. The C++ implementation is part of the final project assignment.
• Simulation of how the code is executed for a graph given by its adjacency list representation (not C++ code, but rather box-and-arrow notation).
• Application to Connectivity problems:
  • Decide if an undirected graph is connected.
  • Decide if there exists a path from a given node v to another node u in a directed graph.
  • Decide if there exists a node that cannot be reached from a node v, in a directed graph.
  • List all the nodes reachable from a given node v.
  Interpretation for computer networks, road networks, etc.