In this tutorial, you will learn what an **adjacency list** is. Additionally, you will discover working instances of adjacency list in C, C++, Java, and Python.

An adjacency list addresses a graph as an array of linked lists.

The index of the array addresses a vertex and every element in its linked list addresses the other vertices that structure an edge with the vertex.

In AdjacencyList, we use an array of a list to represent the graph.

The list size is equivalent to the number of vertex(n).

Let’s assume the list of size n as Adjlist[n]

Adjlist[0] will have all the nodes which are associated with vertex 0.

Adjlist[1] will have all the nodes which are associated with vertex 1, etc.

In this article, you will learn-

## Adjacency List representation

A graph and its identical adjacencylist representation appear beneath.

An adjacency list is proficient in terms of storage since we just need to store the values for the edges. For a sparse graph with a huge number of vertices and edges, this can mean a lot of saved space.

## Adjacency List Structure

The most straightforward adjacency list needs node data structure to store a vertex and graph data structure to coordinate the nodes.

We stay near the fundamental meaning of a graph – an assortment of vertices and edges {V, E}. For simplicity, we use an unlabeled graph rather than a named one for example the vertices are recognized by their files 0,1,2,3.

Let’s dig into the data structures at play here.

struct node { int vertex; struct node* next; }; struct Graph { int numVertices; struct node** adjLists; };

Try not to let the struct node** adjLists overwhelm you.

All we are saying is we need to store a pointer to struct node*. This is on the grounds that we don’t have a clue the number of vertices the graph will have thus we can’t create an array of Linked Lists at compile time.

## Adjacency List C++

It is a similar structure however by using the in-built list STL data structures of C++, we make the structure a piece cleaner. We are additionally ready to abstract the details of the usage.

class Graph { int numVertices; list<int> *adjLists; public: Graph(int V); void addEdge(int src, int dest); };

## Adjacency List Java

We use Java Collections to store the Array of Linked Lists.

class Graph { private int numVertices; private LinkedList<integer> adjLists[]; }

The sort of LinkedList is determined by what data you need to store in it. For a labeled graph, you could store a dictionary rather than an Integer

## Adjacency List Python

There is an explanation Python gets such a lot of affection. A straightforward dictionary of vertices and its edges is a sufficient representation of a graph. You can make the vertex itself as complex as you need.

graph = {'A': set(['B', 'C']), 'B': set(['A', 'D', 'E']), 'C': set(['A', 'F']), 'D': set(['B']), 'E': set(['B', 'F']), 'F': set(['C', 'E'])}

## Python, Java, and C/C++ Examples

**Python**

# Adjascency List representation in Python class AdjNode: def __init__(self, value): self.vertex = value self.next = None class Graph: def __init__(self, num): self.V = num self.graph = [None] * self.V # Add edges def add_edge(self, s, d): node = AdjNode(d) node.next = self.graph[s] self.graph[s] = node node = AdjNode(s) node.next = self.graph[d] self.graph[d] = node # Print the graph def print_agraph(self): for i in range(self.V): print("Vertex " + str(i) + ":", end="") temp = self.graph[i] while temp: print(" -> {}".format(temp.vertex), end="") temp = temp.next print(" \n") if __name__ == "__main__": V = 5 # Create graph and edges graph = Graph(V) graph.add_edge(0, 1) graph.add_edge(0, 2) graph.add_edge(0, 3) graph.add_edge(1, 2) graph.print_agraph()

**Java**

// Adjascency List representation in Java import java.util.*; class Graph { // Add edge static void addEdge(ArrayList<ArrayList<Integer>> am, int s, int d) { am.get(s).add(d); am.get(d).add(s); } public static void main(String[] args) { // Create the graph int V = 5; ArrayList<ArrayList<Integer>> am = new ArrayList<ArrayList<Integer>>(V); for (int i = 0; i < V; i++) am.add(new ArrayList<Integer>()); // Add edges addEdge(am, 0, 1); addEdge(am, 0, 2); addEdge(am, 0, 3); addEdge(am, 1, 2); printGraph(am); } // Print the graph static void printGraph(ArrayList<ArrayList<Integer>> am) { for (int i = 0; i < am.size(); i++) { System.out.println("\nVertex " + i + ":"); for (int j = 0; j < am.get(i).size(); j++) { System.out.print(" -> " + am.get(i).get(j)); } System.out.println(); } } }

**C**

// Adjascency List representation in C #include <stdio.h> #include <stdlib.h> struct node { int vertex; struct node* next; }; struct node* createNode(int); struct Graph { int numVertices; struct node** adjLists; }; // Create a node struct node* createNode(int v) { struct node* newNode = malloc(sizeof(struct node)); newNode->vertex = v; newNode->next = NULL; return newNode; } // Create a graph struct Graph* createAGraph(int vertices) { struct Graph* graph = malloc(sizeof(struct Graph)); graph->numVertices = vertices; graph->adjLists = malloc(vertices * sizeof(struct node*)); int i; for (i = 0; i < vertices; i++) graph->adjLists[i] = NULL; return graph; } // Add edge void addEdge(struct Graph* graph, int s, int d) { // Add edge from s to d struct node* newNode = createNode(d); newNode->next = graph->adjLists[s]; graph->adjLists[s] = newNode; // Add edge from d to s newNode = createNode(s); newNode->next = graph->adjLists[d]; graph->adjLists[d] = newNode; } // Print the graph void printGraph(struct Graph* graph) { int v; for (v = 0; v < graph->numVertices; v++) { struct node* temp = graph->adjLists[v]; printf("\n Vertex %d\n: ", v); while (temp) { printf("%d -> ", temp->vertex); temp = temp->next; } printf("\n"); } } int main() { struct Graph* graph = createAGraph(4); addEdge(graph, 0, 1); addEdge(graph, 0, 2); addEdge(graph, 0, 3); addEdge(graph, 1, 2); printGraph(graph); return 0; }

**C++**

// Adjascency List representation in C++ #include <bits/stdc++.h> using namespace std; // Add edge void addEdge(vector<int> adj[], int s, int d) { adj[s].push_back(d); adj[d].push_back(s); } // Print the graph void printGraph(vector<int> adj[], int V) { for (int d = 0; d < V; ++d) { cout << "\n Vertex " << d << ":"; for (auto x : adj[d]) cout << "-> " << x; printf("\n"); } } int main() { int V = 5; // Create a graph vector<int> adj[V]; // Add edges addEdge(adj, 0, 1); addEdge(adj, 0, 2); addEdge(adj, 0, 3); addEdge(adj, 1, 2); printGraph(adj, V); }

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