### CS201 - Spring 2014 - Class 32

• graphs
- A graph is a set of vertices (or nodes) V and a set of edges (u,v) in E where u, v are in V
- there are different types of graphs (show some examples)
- undirected
- if an edge (u, v) exists (that is u is connected to v) then (v, u) also exits, that is, v is connected to u
- when drawn, we'd draw the edge between u and v as a line
- directed
- edges have a direction, that is, u may be connected to v, but not necessarily vice versa
- when drawn, we'd draw the edge from u to v as an "arrow"
- weighted
- each graph edge has an associated weight
- can have either weighted, undirected graphs or weighted, directed graphs

- graph terminology
- path
- a path is a list of vertices p_1, p_2, ..., p_k where there exists an edge (p_i, p_{i+1}) in E
- intuitively, it's a connection of edges
- a "simple" path is a path where all vertices are unique, except possibly the start and end
- cycle
- a simple path with p_1 = p_k AND where the edges are unique (this avoids the problem of calling A, B, A a cycle in an undirected graph)
- look at cycles in directed and undirected graphs
- a graph is called "acyclic" if it doesn't have any cycles in it
- connected
- a graph is connected if every pair of vertices is connected by a path
- for directed graphs, this is called "strongly connected"
- "weakly connected"?
- if replacing all of the directed edges with undirected edges leaves a connected graph

- where have we seen graphs in this class already?
- tree
- undirected
- acyclic
- note that we have to specify a root

- some other special cases
- twig
- ring
- complete graph
- an edge from every node to every other node
- bipartite graph
- a graph where every vertex can be partitioned into two disjoint sets X and Y, such that all edges connect a vertex u in X to a vertex v in Y

- what are some questions we might want to ask about graphs?
- does it have a cycle?
- is it connected? strongly connected? weakly connected?
- is there a path from a to b?
- what is the shortest path from a to b?
- do graphs have the same structure (these are called isomorphic graphs)?

- where do graphs come up in real life?
- transportation networks (flights, roads, etc.)
- flights
- directed or undirected?
- weighted?
- what are the weights?
- directed or undirected
- weighted?
- what are the weights?
- communication networks
- computer networks
- phone networks
- web
- what are the vertices and edges?
- vertices are web pages
- social networks
- what are the vertices and edges?
- vertices are people
- edges are relationships (e.g. friends on facebook)
- circuit design
- bayesian networks

• representing graphs
- so far, we've drawn them on the board fine, but how are we going to store them for processing?
- each vertex u in V contains a linked list of all the vertices v such that there exists an edge (u, v) in E, that is that there is an edge from u to v

A: B->D
B: A->D
C: D
D: A->B->C->E
E: D

- a |V| by |V| matrix A, such that A_ij is 1 if edge (i, j) is in E, 0 otherwise

A B C D E
A 0 1 0 1 0
B 1 0 0 1 0
C 0 0 0 1 0
D 1 1 1 0 1
E 0 0 0 1 0

- what will this matrix look like if the graph is undirected?
- it will be symmetric
- examples:
- draw the following graphs
-- (undirected)
A: B C
B: A C
C: A B
-- (directed)
A: D
B: D E
C: D B
D: A B C D E
E: B C
-- (directed)
A B C
A 1 0 0
B 0 0 1
C 0 1 0
- how would we incorporate weights into both of these approaches?
- adjacency matrix: store that value in the matrix (instead of just a 0 or a 1)
- What are the benefits/drawbacks of each approach and when might each be useful?
- good for sparse graphs
- more space efficient (for sparse graphs)
- must traverse the adjacency list to discover if an edge exists
- good for dense graphs
- constant time lookup to discover if an edge exists
- for non-weighted graphs, only requires a boolean matrix
- rather than storing adjacent vertices as a linked list, store as a hashtable
- benefits/drawbacks?
- constant time lookup
- fairly space efficient (though some overhead with keeping the table)
- not good for dense graphs

• finding cycles
- given a connected, undirected graph, how can we determine if it has a cycle in it?
- or, given a connected graph, determine that it is not a tree
- what is the definition of a cycle?
- a path, where the endpoints are the same
- idea:
- start at a node, go down a path
- stop when either we find a vertex on the path that we've already seen
- or when we hit a dead-end
- if we hit a dead-end, backtrack and find another path
- if we visit all of the nodes, without finding a repeat vertex, it's acyclic
- does this sound like anything we've seen before?
- depth first search!

- what modifications need to be made?
- if we visit a node that we've already visited, then we've found a cycle
- what about where we just came from?
- need to know where we came from so we can avoid calling that a cycle
- want to return true if we find a cycle, false otherwise