how to implement graph using adjacency list c++ code example

Example 1: graph c++

//Code by Soumyadeep Ghosh insta- @soumyadepp
//linked in :  https://www.linkedin.com/in/soumyadeep-ghosh-90a1951b6/
//Basic implementation of undirected graph using OOP

#include <bits/stdc++.h>

using namespace std;

//undirected graph
class graph
{
  vector<int>*adjacency_list; //array of vectors to store adjacency list
  int Vertices;
  public:
    //constructor
    graph(int n)
    {
      Vertices=n;
      adjacency_list=new vector<int>[Vertices]; //dynamic allocation
    }
  
    void add_edge(int,int);
    void display_graph();
};

int main()
{
  graph g1(5);  //graph of 5 vertices indices- 0 to 4
  //adding edges
  g1.add_edge(0,1); //connect node number 0 to node number 1
  g1.add_edge(1,2); //connect node number 1 to node number 2
  g1.add_edge(1,3); //connect node number 1 to node number 3
  g1.add_edge(2,4); //connect node number 2 to node number 4
  g1.add_edge(2,3); //connect node number 2 to node number 3
  //displaying the graph
  cout<<"The entered Graph is "<<endl;
  g1.display_graph();
  return 0;
}

//function definitions
void graph::add_edge( int u,int v )
   {
     if( u >= Vertices || v >= Vertices )
     {
       cout<<"Overflow"<<endl;
       return;
     }
     if( u<0 || v<0 )
     {
       cout<<"underflow"<<endl;
       return;
     }
     adjacency_list[u].push_back(v);
     adjacency_list[v].push_back(u);
   }
  void graph::display_graph()
   {
     for(int i=0;i<Vertices;i++)
     {
       cout<<"Adjacency list of vertex of vertex "<<i<<endl;
       for(auto it:adjacency_list[i]) //traverse through each list
       {
         cout<<it<<" ";
       }
       cout<<endl;
     }
   }
//thank you!

Example 2: weighted graph c++

//code  by Soumyadeep Ghosh
//insta : @soumyadepp
//linked in: https://www.linkedin.com/in/soumyadeep-ghosh-90a1951b6/

#include <bits/stdc++.h>
using namespace std;

//undirected weighted graph and all functions
class WeightedGraph
{
  vector< pair<int,int> >*adjacency_list;
  int vertices;
  public:
  WeightedGraph(int n)
  {
    vertices=n;
    adjacency_list=new vector< pair<int,int> >[n];
  }
  void add_edge(int v1,int v2,int wt);
  void dfsHelper(int src,bool visited[]);
  void dfs(int src);
  void bfs(int src);
  int minDistance(vector<int>dist,bool visited[]);  
  void djisktra(int src);
  void display_graph();
};

int main()
{
  //graph of five vertices
  WeightedGraph wg1(5);
  //adding edges
  wg1.add_edge(0,1,10);
  wg1.add_edge(1,2,20);
  wg1.add_edge(2,3,30);
  wg1.add_edge(1,3,40);
  wg1.add_edge(2,4,100);
  wg1.add_edge(4,0,10);
  //displaying the graph
  wg1.display_graph();
  //dfs from vertex 0
  wg1.dfs(0);
  //bfs from vertex 0
  wg1.bfs(0);
  //djikstra
  for(int i=0;i<5;i++)
  {
    djikstra(i);
  }
  return 0;
}
//function definitions

void WeightedGraph::add_edge(int v1,int v2,int wt)
{
  /*push the other vertex into the adjacency list of the given vertex
  and vice versa. If it would have been a directed graph,
  only the first line would be enough
  */
  adjacency_list[v1].push_back(make_pair(v2,wt));
  adjacency_list[v2].push_back(make_pair(v1,wt));
}

void WeightedGraph::dfsHelper(int src,bool visited[])
{
  visited[src]=true;
  cout<<src<<" ";
  for(vector<int>::iterator it=adjacency_list.begin();i!=adjacency_list.end();it++)
  {
    if(!visited[it->first]);
      dfsHelper(it->first,visited);
  }
}
void WeightedGraph::dfs(int src)
{
  bool visited[vertices];
  for(int i=0;i<vertices;i++)
    visited[i]=false;
  dfsHelper(src,visited);
}
void WeightedGraph::bfs(int src)
{
  bool visited[vertices];
  for(int i=0;i<vertices;i++)
    visited[i]=false;
  cout<<src<<" ";
  visited[src]=true;
  queue<int>helper;
  helper.push(src);
  while(!helper.empty())
  {
    src=helper.front();
    for(vector<int>::iterator it=adjacency_list[src].begin();it!+adjacency_list[src].end();it++)
    {
      if(!visited[it->first])
      {
        visited[it->first]=true;
        cout<<it->first<<" ";
        helper.push(it->first);
      }
    }
    helper.pop();
  }
}

int WeightedGraph::minDistance(vector<int>dist,bool visited[])
  {
    int min=INT_MAX;
    int minIndex=INT_MAX;
    for(int i=0;i<N;i++)
    {
      if(!visited[i]&&dist[i]<=min)
      {
        min=dist[i];
        minIndex=i;
      }
    }
    return minIndex;
  }


void WeightedGraph::djikstra(int src)
  {
    vector<int>dist;
    bool visited[vertices];
    for(int i=0;i<vertices;i++)
    {
      dist.push_back(INT_MAX);
      visited[i]=false;
    }
    visited[src]=true;
    dist[src]=0;
    for(int i=0;i<vertices-1;i++)
    {
      int k=minDistance(dist,visited);
      visited[k]=true;
      for(int j=0;j<vertices;j++)
      {
        if(!visited[i]&&dist[i]!=INT_MAX&&adjacency_list[i][j].second+dist[i]<dist[j])
        {
          dist[j]=adjacency_list[i][j].second+dist[i];
        }
      }
    }
    for(int i=0;i<dist.size();i++)
      cout<<dist[i]<<" ";
    
    cout<<endl;
  }
void WeightedGraph::display_graph()
{
  int a,b;
  //first loop to traverse across vertices
  for(int i=0;i<vertices;i++)
  {
      cout<<"Adjacency list of vertex "<<i<<endl;
    //second loop to traverse across the adjacency list of some vertex i
    for(auto it=adjacency_list[i].begin();it!=adjacency_list[i].end();it++)
    {
      //set a as the vertex number and b as the weight
      a=it->first;
      b=it->second;
      cout<<"Vertex : "<<a<<" Weight : "<<b<<endl;
    }
    cout<<endl;
  }
}

//thank you!