Graph implementation C++

Question

I was wondering about a quick to write implementation of a graph in c++. I need the data structure to be easy to manipulate and use graph algorithms(such as BFS,DFS, Kruskal

Answer 1

Here is a basic implementation of a graph. Note: I use vertex which is chained to next vertex. And each vertex has a list pointing to adjacent nodes.

#include <iostream>
using namespace std;


// 1 ->2 
// 1->4
// 2 ->3
// 4->3
// 4 -> 5
// Adjacency list
// 1->2->3-null
// 2->3->null
//4->5->null;

// Structure of a vertex
struct vertex {
   int i;
   struct node *list;
   struct vertex *next;
};
typedef struct vertex * VPTR;

// Struct of adjacency list
struct node {
    struct vertex * n;
    struct node *next;
};

typedef struct node * NODEPTR;

class Graph {
    public:
        // list of nodes chained together
        VPTR V;
        Graph() {
            V = NULL;
        }
        void addEdge(int, int);
        VPTR  addVertex(int);
        VPTR existVertex(int i);
        void listVertex();
};

// If vertex exist, it returns its pointer else returns NULL
VPTR Graph::existVertex(int i) {
    VPTR temp  = V;
    while(temp != NULL) {
        if(temp->i == i) {
            return temp;
        }
        temp = temp->next;
    }
   return NULL;
}
// Add a new vertex to the end of the vertex list
VPTR Graph::addVertex(int i) {
    VPTR temp = new(struct vertex);
    temp->list = NULL;
    temp->i = i;
    temp->next = NULL;

    VPTR *curr = &V;
    while(*curr) {
        curr = &(*curr)->next;
    }
    *curr = temp;
    return temp;
}

// Add a node from vertex i to j. 
// first check if i and j exists. If not first add the vertex
// and then add entry of j into adjacency list of i
void Graph::addEdge(int i, int j) {

    VPTR v_i = existVertex(i);   
    VPTR v_j = existVertex(j);   
    if(v_i == NULL) {
        v_i = addVertex(i);
    }
    if(v_j == NULL) {
        v_j = addVertex(j);
    }

    NODEPTR *temp = &(v_i->list);
    while(*temp) {
        temp = &(*temp)->next;
    }
    *temp = new(struct node);
    (*temp)->n = v_j;
    (*temp)->next = NULL;
}
// List all the vertex.
void Graph::listVertex() {
    VPTR temp = V;
    while(temp) {
        cout <<temp->i <<" ";
        temp = temp->next;
    }
    cout <<"\n";

}

// Client program
int main() {
    Graph G;
    G.addEdge(1, 2);
    G.listVertex();

}

With the above code, you can expand to do DFS/BFS etc.