Graph Data Structure

HARAMAYA UNIVERSITY
SCHOOL OF GRADUATE STUDIES
DEPARTMENT OF COMPUTER SCIENCE
GRAPH DATA STRUCTURE
By: Keno Benti
ID: Sgs/0645/11
JAN 10,2019

Overview
• A Graph is a non-linear data structure, which consists of vertices(or
nodes) connected by edges(or arcs) where edges may be directed
or undirected.
• Graphs are a powerful and versatile data structure that easily allow
you to represent real life relationships between different types of
data (nodes). There are two main parts of a graph:
 The vertices (nodes) where the data is stored
The edges (connections) which connect the nodes
• Vertices i and j are adjacent vertices iff (i, j) is an edge in the
graph

Graph and Its Representations
• The choice of graph representation is said to be situation
specific. It totally depends on the type of operations to be
performed and ease of use.
• Following the most commonly used representations of a graph.
1. Adjacency Matrix
2. Adjacency List

Adjacency Matrix
 is a 2D array of size V x V where V is the number of vertices in a graph. Let the
2D array be adj[][], a slot adj[i][j] = 1 indicates that there is an edge from
vertex i to vertex j.
 Between each pair of vertices, cost of one edge is to be stored. This
shows which vertices are adjacent to one another.
For undirected graphs half of the graph is repeated information. Hence these
matrices are said to be space inefficient.
The memory use of an adjacency matrix is O(n^2) where n = number of
vertices.

Advantages of using adjacency matrix are as follows:
 Easy to understand, implement and convenient to work with.
 Removing an edge involves time complexity of O(1).
 Queries like whether there is an edge from vertex „u‟ to
vertex „v‟ are efficient and require time complexity, O(1).

Disadvantages of using adjacency matrix are as follows:
 Consumes more space O(V^2). Even if the graph is sparse(contains
less number of edges), it consumes the same space. Adding a
vertex is O(V^2) time.
 If the number of nodes in the graph may change, matrix
representation is too inflexible (especially if we don‟t know the
maximal size of the graph).

Adjacency List
• In this representation, every vertex of graph contains list of its adjacent
vertices.
Advantages of using adjacency list are as follows:
• Addition of a vertex and connecting new vertices with the existing ones
is easier.
• Has a space complexity of O(|V|+|E|).
• It allows us to store graph in more compact form and to get the list of
adjacent vertices in O(1) time which is a big benefit for some algorithms.

Cont’d…
Disadvantages of using adjacency list are as follows:
• Queries like whether there is an edge from vertex u to vertex v are not
efficient and require time complexity, O(V).
• It does not allow us to make an efficient implementation, if dynamic
change of vertices number is required.

Types Of Graphs
1. Undirected Graph all edges are undirected or If the connection is
symmetric (in other words A is connected to B ↔ B is connected to
A), then we say the graph is undirected.
2. Directed Graph all edges are directed or If an edge only implies
one direction of connection, we say the graph is directed.
3. Weighted Graph: A weighted graph is a graph where each edge
has an associated numerical value, called weight.

Undirected Graphs
 The pair of vertices representing any edge is unordered. Thus, the
pairs (u,v) and (v,u) represent the same edge.
Example: a graph G2
• V(G2)={0,1,2,3,4,5,6}
• E( G2)={(0,1),(0,2), (1,3),(1,4),(2,5),(2,6)}
• G2 is also a tree that is a special case of graph
• An undirected graph is said to be connected if there is a path
between every pair of vertices in the graph.

Directed Graphs (Digraph)
 each edge is represented by a ordered pairs <u,v>
Example: a graph G3
• V(G3)={0,1,2}
• E(G3)={<0,1>,<1,0>,<1,2>}
• A directed path in a directed graph G is a sequence of distinct
vertices, such that there is an edge from each vertex in the
sequence to the next.

Weighted Graph
• A weighted graph is a graph where each edge has an
associated numerical value, called weight. Weighted graphs
may be either directed or undirected. The weight of the edge
is often referred to as the “cost” of the edge. Example of
weighted graph is:

Graph Traversal Algorithms
Graph traversal means visiting all the nodes of the
graph.
We want to visit every vertex and every edge exactly
once in some well-defined order.
There are two primary traversal algorithms:
• Breadth-First Search (BFS) and
• Depth-First Search (DFS).

DFS/BFS Search Algorithm
SEARCH(Node n):
1. Visit n.
2. Put n's children on a list (Stack or Queue).
3. For each child c in list:
SEARCH(c)
• If we use a Stack and recursive, we get DFS
• If we use a Queue and iterative, we get BFS

BFS-Breadth First Search
 Idea: Visit all children before first grandchild.
• Visiting order: Visit a vertex, then visit all of its neighbors,
then visit all of the neighbors of its neighbors, . . .
• BFS is that a BFS is best used at finding the shortest paths in a given
Graph
• Needs flags or someway to preventing infinite loops:
Weakness

BFS Algorithm
create a queue Q
mark v as visited and put v into Q
while Q is non-empty
remove the head u of Q
mark and enqueue all (unvisited) neighbours of u

DFS-Depth First Search
• Idea: Follow a single path to its end, then backtrack
• Is an algorithm for traversing or searching a tree, tree
structure or graph.
• One starts at the root (selecting some node as the root in the
graph case) and explores as far as possible along each branch
before backtracking.

Graph Traversal Algorithm Complexity
Time Complexity
• BFS Vs DFS Time complexity : O(V+E), when implemented
using the adjacency list. Where V is number of vertices in the
graph and E is number of edges in the graph.
• Space complexity (memory)
BFS :Space complexity is O(|V|) as well - since at worst case
you need to hold all vertices in the queue.

Cont’d…
• DFS: Space Complexity depends on the implementation, a
recursive implementation can have a O(h) space complexity
[worst case], where h is the maximal depth of your tree. But
Depth-first can waste time going down a “rabbit hole”, when
solution is high in tree.
• Worst Case for DFS will be the best case for BFS, and the Best
Case for DFS will be the worst case for BFS.

Conclusion
• A Graph is a non-linear data structure, which consists of vertices(or
nodes) connected by edges(or arcs) where edges may be directed or
undirected.
• Graph is represented by two important ways: Adjacency List and
Adjacency Matrix.
• Graph traversal is a process of checking or updating each vertex in a
graph.
• Graph traversal means visiting each and exactly one node.
• There are two graph traversals: Breadth first Search: Visits the neighbor
vertices before visiting the child vertices and Depth First Search:is used
for traversing a finite graph.

Recommendation
• I recommend for peoples to use hybrid (both BFS and DFS) approach. Because
Worst Case for DFS will be the best case for BFS, and the Best Case for DFS will
be the worst case for BFS in terms of space complexity.
• Building a good general-purpose graph type is surprisingly tricky and difficult.
For this reason, I suggest that you check out existing implementations
(particularly LEDA) before hacking up your own. Note that it costs only time
linear in the size of the larger data structure to convert between adjacency
matrices and adjacency lists. This conversion is unlikely to be the bottleneck in
any application, if you decide you want to use both data structures and have
the space to store them. This usually isn't necessary but might prove simplest if
you are confused about the alternatives.

References
1. Thomas H. Cormen, Charles E. Leirserson, Ronald L. Rivest, Introduction
to algorithms, Cambridge, Massachusetts London, England 2000
2. Manber, U. (1989), Introduction to Algorithms|Creative
Approach, Addison-Wesley, Reading, Massachusetts
3. Danny Sleator, “Parallel and Sequential Data Structures and
Algorithms,15-210 (fall 2013) ”, Sept. 24 , 2013
4. https://www.tutorialspoint.com/data_structures_algorithms/graph_data
_structure.htm
5. https://www.geeksforgeeks.org/graph-and-its-representations/
6. https://hackernoon.com/graphs-in-cs-and-its-traversal-algorithms-
cfee5533f74e
7. https://www.hackerearth.com/practice/algorithms/graphs/depth-first-
search/tutorial/

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