# Connected Components in an Undirected Graph

• Difficulty Level : Medium
• Last Updated : 13 Sep, 2022

Given an undirected graph, the task is to print all the connected components line by line.

Examples:

Input: Consider the following graph

Example of an undirected graph

Output:
0 1 2
3 4
Explanation: There are 2 different connected components.
They are {0, 1, 2} and {3, 4}.

Recommended Practice

We have discussed algorithms for finding strongly connected components in directed graphs in following posts.
Kosarajuâ€™s algorithm for strongly connected components
Tarjanâ€™s Algorithm to find Strongly Connected Components

## Connected Components for undirected graph using DFS:

Finding connected components for an undirected graph is an easier task. The idea is to

Do either BFS or DFS starting from every unvisited vertex, and we get all strongly connected components.

Follow the steps mentioned below to implement the idea using DFS:

• Initialize all vertices as not visited.
• Do the following for every vertex v:
• If v is not visited before, call the DFS. and print the newline character to print each component in a new line
• Mark v as visited and print v.
• For every adjacent u of v, If u is not visited, then recursively call the DFS.

Below is the implementation of above algorithm.

## C++

 `// C++ program to print connected components in``// an undirected graph``#include ``using` `namespace` `std;` `// Graph class represents a undirected graph``// using adjacency list representation``class` `Graph {``    ``int` `V; ``// No. of vertices` `    ``// Pointer to an array containing adjacency lists``    ``list<``int``>* adj;` `    ``// A function used by DFS``    ``void` `DFSUtil(``int` `v, ``bool` `visited[]);` `public``:``    ``Graph(``int` `V); ``// Constructor``    ``~Graph();``    ``void` `addEdge(``int` `v, ``int` `w);``    ``void` `connectedComponents();``};` `// Method to print connected components in an``// undirected graph``void` `Graph::connectedComponents()``{``    ``// Mark all the vertices as not visited``    ``bool``* visited = ``new` `bool``[V];``    ``for` `(``int` `v = 0; v < V; v++)``        ``visited[v] = ``false``;` `    ``for` `(``int` `v = 0; v < V; v++) {``        ``if` `(visited[v] == ``false``) {``            ``// print all reachable vertices``            ``// from v``            ``DFSUtil(v, visited);` `            ``cout << ``"\n"``;``        ``}``    ``}``    ``delete``[] visited;``}` `void` `Graph::DFSUtil(``int` `v, ``bool` `visited[])``{``    ``// Mark the current node as visited and print it``    ``visited[v] = ``true``;``    ``cout << v << ``" "``;` `    ``// Recur for all the vertices``    ``// adjacent to this vertex``    ``list<``int``>::iterator i;``    ``for` `(i = adj[v].begin(); i != adj[v].end(); ++i)``        ``if` `(!visited[*i])``            ``DFSUtil(*i, visited);``}` `Graph::Graph(``int` `V)``{``    ``this``->V = V;``    ``adj = ``new` `list<``int``>[V];``}` `Graph::~Graph() { ``delete``[] adj; }` `// method to add an undirected edge``void` `Graph::addEdge(``int` `v, ``int` `w)``{``    ``adj[v].push_back(w);``    ``adj[w].push_back(v);``}` `// Driver code``int` `main()``{``    ``// Create a graph given in the above diagram``    ``Graph g(5); ``// 5 vertices numbered from 0 to 4``    ``g.addEdge(1, 0);``    ``g.addEdge(2, 1);``    ``g.addEdge(3, 4);` `    ``cout << ``"Following are connected components \n"``;``    ``g.connectedComponents();` `    ``return` `0;``}`

## Java

 `// Java program to print connected components in``// an undirected graph``import` `java.util.ArrayList;``class` `Graph {``    ``// A user define class to represent a graph.``    ``// A graph is an array of adjacency lists.``    ``// Size of array will be V (number of vertices``    ``// in graph)``    ``int` `V;``    ``ArrayList > adjListArray;` `    ``// constructor``    ``Graph(``int` `V)``    ``{``        ``this``.V = V;``        ``// define the size of array as``        ``// number of vertices``        ``adjListArray = ``new` `ArrayList<>();` `        ``// Create a new list for each vertex``        ``// such that adjacent nodes can be stored` `        ``for` `(``int` `i = ``0``; i < V; i++) {``            ``adjListArray.add(i, ``new` `ArrayList<>());``        ``}``    ``}` `    ``// Adds an edge to an undirected graph``    ``void` `addEdge(``int` `src, ``int` `dest)``    ``{``        ``// Add an edge from src to dest.``        ``adjListArray.get(src).add(dest);` `        ``// Since graph is undirected, add an edge from dest``        ``// to src also``        ``adjListArray.get(dest).add(src);``    ``}` `    ``void` `DFSUtil(``int` `v, ``boolean``[] visited)``    ``{``        ``// Mark the current node as visited and print it``        ``visited[v] = ``true``;``        ``System.out.print(v + ``" "``);``        ``// Recur for all the vertices``        ``// adjacent to this vertex``        ``for` `(``int` `x : adjListArray.get(v)) {``            ``if` `(!visited[x])``                ``DFSUtil(x, visited);``        ``}``    ``}``    ``void` `connectedComponents()``    ``{``        ``// Mark all the vertices as not visited``        ``boolean``[] visited = ``new` `boolean``[V];``        ``for` `(``int` `v = ``0``; v < V; ++v) {``            ``if` `(!visited[v]) {``                ``// print all reachable vertices``                ``// from v``                ``DFSUtil(v, visited);``                ``System.out.println();``            ``}``        ``}``    ``}` `    ``// Driver code``    ``public` `static` `void` `main(String[] args)``    ``{``        ``// Create a graph given in the above diagram``        ``Graph g = ``new` `Graph(``5``);` `        ``g.addEdge(``1``, ``0``);``        ``g.addEdge(``2``, ``1``);``        ``g.addEdge(``3``, ``4``);``        ``System.out.println(``            ``"Following are connected components"``);``        ``g.connectedComponents();``    ``}``}`

## Python3

 `# Python program to print connected``# components in an undirected graph`  `class` `Graph:` `    ``# init function to declare class variables``    ``def` `__init__(``self``, V):``        ``self``.V ``=` `V``        ``self``.adj ``=` `[[] ``for` `i ``in` `range``(V)]` `    ``def` `DFSUtil(``self``, temp, v, visited):` `        ``# Mark the current vertex as visited``        ``visited[v] ``=` `True` `        ``# Store the vertex to list``        ``temp.append(v)` `        ``# Repeat for all vertices adjacent``        ``# to this vertex v``        ``for` `i ``in` `self``.adj[v]:``            ``if` `visited[i] ``=``=` `False``:` `                ``# Update the list``                ``temp ``=` `self``.DFSUtil(temp, i, visited)``        ``return` `temp` `    ``# method to add an undirected edge``    ``def` `addEdge(``self``, v, w):``        ``self``.adj[v].append(w)``        ``self``.adj[w].append(v)` `    ``# Method to retrieve connected components``    ``# in an undirected graph``    ``def` `connectedComponents(``self``):``        ``visited ``=` `[]``        ``cc ``=` `[]``        ``for` `i ``in` `range``(``self``.V):``            ``visited.append(``False``)``        ``for` `v ``in` `range``(``self``.V):``            ``if` `visited[v] ``=``=` `False``:``                ``temp ``=` `[]``                ``cc.append(``self``.DFSUtil(temp, v, visited))``        ``return` `cc`  `# Driver Code``if` `__name__ ``=``=` `"__main__"``:` `    ``# Create a graph given in the above diagram``    ``# 5 vertices numbered from 0 to 4``    ``g ``=` `Graph(``5``)``    ``g.addEdge(``1``, ``0``)``    ``g.addEdge(``2``, ``1``)``    ``g.addEdge(``3``, ``4``)``    ``cc ``=` `g.connectedComponents()``    ``print``(``"Following are connected components"``)``    ``print``(cc)` `# This code is contributed by Abhishek Valsan`

## Javascript

 ``

Output

```Following are connected components
0 1 2
3 4 ```

Time Complexity: O(V + E) where V is the number of vertices and E is the number of edges.
Auxiliary Space: O(V)

## Connected Component for undirected graph using Disjoint Set Union:

The idea to solve the problem using DSU (Disjoint Set Union) is

Initially declare all the nodes as individual subsets and then visit them. When a new unvisited node is encountered, unite it with the under. In this manner, a single component will be visited in each traversal.

Follow the below steps to implement the idea:

• Declare an array arr[] of size V where V is the total number of nodes.
• For every index i of array arr[], the value denotes who the parent of ith vertex is.
• Initialise every node as the parent of itself and then while adding them together, change their parents accordingly.
• Traverse the nodes from 0 to V:
• For each node that is the parent of itself start the DSU.
• Print the nodes of that disjoint set as they belong to one component.

Below is the implementation of the above approach.

## C++

 `#include ``using` `namespace` `std;` `int` `merge(``int``* parent, ``int` `x)``{``    ``if` `(parent[x] == x)``        ``return` `x;``    ``return` `merge(parent, parent[x]);``}` `int` `connectedcomponents(``int` `n, vector >& edges)``{``    ``int` `parent[n];``    ``for` `(``int` `i = 0; i < n; i++) {``        ``parent[i] = i;``    ``}``    ``for` `(``auto` `x : edges) {``        ``parent[merge(parent, x[0])] = merge(parent, x[1]);``    ``}``    ``int` `ans = 0;``    ``for` `(``int` `i = 0; i < n; i++) {``        ``ans += (parent[i] == i);``    ``}``    ``for` `(``int` `i = 0; i < n; i++) {``        ``parent[i] = merge(parent, parent[i]);``    ``}``    ``map<``int``, list<``int``> > m;``    ``for` `(``int` `i = 0; i < n; i++) {``        ``m[parent[i]].push_back(i);``    ``}``    ``for` `(``auto` `it = m.begin(); it != m.end(); it++) {``        ``list<``int``> l = it->second;``        ``for` `(``auto` `x : l) {``            ``cout << x << ``" "``;``        ``}``        ``cout << endl;``    ``}``    ``return` `ans;``}` `int` `main()``{``    ``int` `n = 5;``    ``vector<``int``> e1 = { 0, 1 };``    ``vector<``int``> e2 = { 2, 1 };``    ``vector<``int``> e3 = { 3, 4 };``    ``vector > e;``    ``e.push_back(e1);``    ``e.push_back(e2);``    ``e.push_back(e3);` `    ``cout << ``"Following are connected components:\n"``;``    ``int` `a = connectedcomponents(n, e);``    ``return` `0;``}`

Output

```Following are connected components:
0 1 2
3 4 ```

Time Complexity: O(V)
Auxiliary Space: O(V)

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