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We're a place where coders share, stay up-to-date and grow their careers. # Representation of Graph

Hello, in this post we'll discuss the representations of a graph, their characteristics, space complexity, and also their implementation in python.

## Representation of Graph

### 1. Adjacency matrix

in this type of representation, we use 2-dimensional arrays to represent the graph where the number of columns and rows is the total number of vertices.

• if `A[i][j] = 1` that means i and j are adjacent.

### characteristics of using adjacency matrix

• Fast to lookup and check for presence or absence of a specific edge between any two nodes O(1)
• Fast to add a new edge O(1)
• Slow to iterate over all edges
• Slow to add or delete a node O(n2)

### Space complexity of adjacency matrix

• The space complexity of adjacency matrix is O(n2)

### Example of the adjacency matrix ### Implementation Of Adjacency matrix in python

``````class Graph():
def __init__(self, matrixSize):
# fill the matrix with 0.
for i in range(matrixSize):
self.adjacencyMatrix.append([0 for i in range(matrixSize)])
self.matrixSize = matrixSize

def addEdge(self, node1, node2):

def deleteEdge(self, node1, node2):
# if there is an edge between the two giving nodes
if self.adjacencyMatrix[node1][node2] == 1 :
``````

### 2. Adjacency List

The adjacency list is represented as an array of linked lists, where each index of the array represents a node and each node represents its adjacents nodes using a linked list.

### characteristics of adjacency list

• Memory usage depends on the number of edges (not the number of nodes) which might save a lot of memory
• slower than matrix for checking the presence or the absence of an edges
• Fast to iterate over all edges

### Space of complexity of adjacency list

The space complexity of the adjacency list is O(|V|+|E|).

### Example of adjacency list ### Implementation of adjacency list

``````class AdjNode:
def __init__(self, data):
self.vertex = data
self.next = None

# A class to represent a graph. A graph
# is the list of the adjacency lists.
# Size of the array will be the no. of the
# vertices "V"
class Graph:
def __init__(self, vertices):
self.V = vertices
self.graph = [None] * self.V

# Function to add an edge in an undirected graph
def add_edge(self, src, dest):
# Adding the node to the source node
node.next = self.graph[src]
self.graph[src] = node

# Adding the source node to the destination as
# it is the undirected graph
node.next = self.graph[dest]
self.graph[dest] = node

# Function to print the graph
def print_graph(self):
for i in range(self.V):
print("Adjacency list of vertex {}\n head".format(i), end="")
temp = self.graph[i]
while temp:
print(" -> {}".format(temp.vertex), end="")
temp = temp.next
print(" \n")
``````