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Graph implementation using STL for competitive programming | Set 1 (DFS of Unweighted and Undirected)
• Difficulty Level : Medium
• Last Updated : 19 Mar, 2021

We have introduced Graph basics in Graph and its representations. In this post, a different STL based representation is used that can be helpful to quickly implement graph using vectors. The implementation is for adjacency list representation of graph.
Following is an example undirected and unweighted graph with 5 vertices. Below is adjacency list representation of the graph. We use vector in STL to implement graph using adjacency list representation.

• vector : A sequence container. Here we use it to store adjacency lists of all vertices. We use vertex number as index in this vector.

The idea is to represent graph as an array of vectors such that every vector represents adjacency list of a vertex. Below is complete STL based C++ program for DFS Traversal

## C++

 `// A simple representation of graph using STL,``// for the purpose of competitive programming``#include``using` `namespace` `std;` `// A utility function to add an edge in an``// undirected graph.``void` `addEdge(vector<``int``> adj[], ``int` `u, ``int` `v)``{``    ``adj[u].push_back(v);``    ``adj[v].push_back(u);``}` `// A utility function to do DFS of graph``// recursively from a given vertex u.``void` `DFSUtil(``int` `u, vector<``int``> adj[],``                    ``vector<``bool``> &visited)``{``    ``visited[u] = ``true``;``    ``cout << u << ``" "``;``    ``for` `(``int` `i=0; i adj[], ``int` `V)``{``    ``vector<``bool``> visited(V, ``false``);``    ``for` `(``int` `u=0; u *adj = new vector[V];``    ``vector<``int``> adj[V];` `    ``// Vertex numbers should be from 0 to 4.``    ``addEdge(adj, 0, 1);``    ``addEdge(adj, 0, 4);``    ``addEdge(adj, 1, 2);``    ``addEdge(adj, 1, 3);``    ``addEdge(adj, 1, 4);``    ``addEdge(adj, 2, 3);``    ``addEdge(adj, 3, 4);``    ``DFS(adj, V);``    ``return` `0;``}`

## Python3

 `# A simple representation of graph using STL,``# for the purpose of competitive programming` `# A utility function to add an edge in an``# undirected graph.``def` `addEdge(adj, u, v):``    ``adj[u].append(v)``    ``adj[v].append(u)``    ``return` `adj` `# A utility function to do DFS of graph``# recursively from a given vertex u.``def` `DFSUtil(u, adj, visited):``    ``visited[u] ``=` `True``    ``print``(u, end ``=` `" "``)``    ``for` `i ``in` `range``(``len``(adj[u])):``        ``if` `(visited[adj[u][i]] ``=``=` `False``):``            ``DFSUtil(adj[u][i], adj, visited)` `# This function does DFSUtil() for all``# unvisited vertices.``def` `DFS(adj, V):``    ``visited ``=` `[``False``]``*``(V``+``1``)` `    ``for` `u ``in` `range``(V):``        ``if` `(visited[u] ``=``=` `False``):``            ``DFSUtil(u, adj, visited)` `# Driver code``if` `__name__ ``=``=` `'__main__'``:``    ``V ``=` `5` `    ``# The below line may not work on all``    ``# compilers.  If it does not work on``    ``# your compiler, please replace it with``    ``# following``    ``# vector *adj = new vector[V]``    ``adj ``=` `[[] ``for` `i ``in` `range``(V)]` `    ``# Vertex numbers should be from 0 to 4.``    ``adj ``=` `addEdge(adj, ``0``, ``1``)``    ``adj ``=` `addEdge(adj, ``0``, ``4``)``    ``adj ``=` `addEdge(adj, ``1``, ``2``)``    ``adj ``=` `addEdge(adj, ``1``, ``3``)``    ``adj ``=` `addEdge(adj, ``1``, ``4``)``    ``adj ``=` `addEdge(adj, ``2``, ``3``)``    ``adj ``=` `addEdge(adj, ``3``, ``4``)``    ``DFS(adj, V)` `# This code is contributed by mohit kumar 29.`

Output :

`0 1 2 3 4`

Below are related articles:
Graph implementation using STL for competitive programming | Set 2 (Weighted graph)
Dijkstra’s Shortest Path Algorithm using priority_queue of STL
Dijkstra’s shortest path algorithm using set in STL
Kruskal’s Minimum Spanning Tree using STL in C++
Prim’s algorithm using priority_queue in STL
This article is contributed by Shubham Gupta. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.