Basic Graph Algorithms

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Basic Graph Algorithms. Programming Puzzles and Competitions CIS 4900 / 5920 Spring 2009. Outline. Introduction/review of graphs Some basic graph problems &amp; algorithms Start of an example question from ICPC’07 (“ Tunnels ”). Relation to Contests.

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### Basic Graph Algorithms

Programming Puzzles and Competitions

CIS 4900 / 5920

Spring 2009

Outline
• Introduction/review of graphs
• Some basic graph problems & algorithms
• Start of an example question from ICPC’07 (“Tunnels”)
Relation to Contests
• Many programming contest problems can be viewed as graph problems.
• Some graph algorithms are complicated, but a few are very simple.
• If you can find a way to apply one of these, you will do well.
How short & simple?

int [][] path = new int[edge.length][edge.length];

for (int i =0; i < n; i++)

for (int j = 0; j < n; j++)

path[i][j] = edge[i][j];

for (int k = 0; k < n; k++)

for (int i =0; i < n; i++)

for (int j = 0; j < n; j++)

if (path[i][k] != 0 && path[k,j] != 0) {

x = path[i][k] + path[k][j];

if ((path[i,j] == 0) || path[i][j] > x) path[i][j] = x;

}

Directed Graphs
• G = (V, E)
• V = set of vertices (a.k.a. nodes)
• E = set of edges (ordered pairs of nodes)
Directed Graph
• V = { a, b, c, d }
• E = { (a, b), (c, d), (a, c), (b, d), (b, c) }

b

d

a

c

Undirected Graph
• V = { a, b, c, d }
• E = { {a, b}, {c, d}, {a, c}, {b, d}, {b, c} }

b

d

a

c

Undirected Graph as Directed
• V = { a, b, c, d }
• E = { (a, b), (b,a),(c,d),(d,c),(a,c),(c,a), (b,d),(d,b),(b,c)(c,b)}

b

d

a

c

Can also be viewed as symmetric directed graph, replacingeach undirected edge by a pair of directed edges.

Computer Representations
• Edge list
• Hash table of edges
Edge List

1

3

0

2

Often corresponds to the input format for contest problems.

Container (set) of edges may be used by

with two

arrays:

1

3

0

with pointers

& dynamic

allocation:

2

Can save space and time if graph is sparse.

Hash Table (Associative Map)

H(0,1)

H(1,2)

etc.

1

1

3

0

1

2

good for storing information about nodes or edges, e.g., edge weight

A[i][j] = 1 → (i,j) i EA[i][j] = 0 otherwise

1

3

0

2

a very convenient representation for simple coding of algorithms,although it may waste time & space if the graph is sparse.

Some Basic Graph Problems
• Connectivity, shortest/longest path
• Single source
• All pairs: Floyd-Warshall Algorithm
• dynamic programming, efficient, very simple
• MaxFlow (MinCut)
• Iterative flow-pushing algorithms
Floyd-Warshall Algorithm

Assume edgeCost(i,j) returns the cost of the edge from i to j (infinity if there is none), n is the number of vertices, and edgeCost(i,i) = 0

intpath[][]; // a 2-D matrix.

// At each step, path[i][j] is the (cost of the) shortest path

// from i to jusing intermediate vertices (1..k-1).

// Each path[i][j] is initialized to edgeCost (i,j)

// or ∞ if there is no edge between i and j.

procedureFloydWarshall ()

for k in 1..n

for each pair (i,j) in {1,..,n}x{1,..,n}

path[i][j] = min ( path[i][j], path[i][k]+path[k][j] );

* Time complexity: O(|V|3 ).

Details
• Need some value to represent pairs of nodes that are not connected.
• If you are using floating point, there is a value ∞ for which arithmetic works correctly.
• But for most graph problems you may want to use integer arithmetic.
• Choosing a good value may simplify code

When and why to use F.P. vs. integers is an interesting side discussion.

Example

Suppose we use path[i][j] == 0 to indicate lack of connection.

if (path[i][k] != 0 && path[k,j] != 0) {

x = path[i][k] + path[k][j];

if ((path[i,j] == 0) || path[i][j] > x) path[i][j] = x;

}

How it works

path[i][j]

i

j

paths that go

though only

nodes 0..k-1

path[i][k]

path[k,j]

k

Correction

In class, I claimed that this algorithm could be adapted to find length of longest cycle-free path, and to count cycle-free paths.

That is not true.

However there is a generalization to find the maximum flow between points, and the maximum-flow path:

for k in 1,..,n

for each pair (i,j) in {1,..,n}x{1,..,n}

maxflow[i][j] = max (maxflow[i][j]

min (maxflow[i][k], maxflow[k][j]);