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.
adjacency_list_representation

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

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

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