# Graph implementation using STL for competitive programming | Set 1 (DFS of Unweighted and Undirected)

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 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.

```// A simple representation of graph using STL,
// for the purpose of competitive programming
#include<bits/stdc++.h>
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[u].size(); i++)
if (visited[adj[u][i]] == false)
DFSUtil(adj[u][i], adj, visited);
}

// This function does DFSUtil() for all
// unvisited vertices.
void DFS(vector<int> adj[], int V)
{
vector<bool> visited(V, false);
for (int u=0; u<V; u++)
if (visited[u] == false)
DFSUtil(u, adj, visited);
}

// Driver code
int main()
{
int V = 5;
vector<int> adj[V];
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;
}
```

Output :

`0 1 2 3 4`

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.

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