C++ Program to Check if an Undirected Graph is a Tree or Not Using DFS

Code:

#include   iostream
#include   list
#include   limits.h
using namespace std;

// Class for an undirected graph
class Graph
{
        int V; // No. of vertices
        list *adj; // Pointer to an array containing adjacency lists
        bool isCyclicUtil(int v, bool visited[], int parent);
    public:
        Graph(int V); // Constructor
        void addEdge(int v, int w); // to add an edge to graph
        bool isCyclic(); // returns true if there is a cycle
};

Graph::Graph(int V)
{
    this->V = V;
    adj = new list [V];
}

void Graph::addEdge(int v, int w)
{
    adj[v].push_back(w); // Add w to v’s list.
    adj[w].push_back(v); // Add v to w’s list.
}

// A recursive function that uses visited[] and parent to detect
// cycle in subgraph reachable from vertex v.
bool Graph::isCyclicUtil(int v, bool visited[], int parent)
{
    // Mark the current node as visited
    visited[v] = true;

    // Recur for all the vertices adjacent to this vertex
    list::iterator i;
    for (i = adj[v].begin(); i != adj[v].end(); ++i)
    {
        // If an adjacent is not visited, then recur for that adjacent
        if (!visited[*i])
        {
            if (isCyclicUtil(*i, visited, v))
                return true;
        }

        // If an adjacent is visited and not parent of current vertex,
        // then there is a cycle.
        else if (*i != parent)
            return true;
    }
    return false;
}

// Returns true if the graph contains a cycle, else false.
bool Graph::isCyclic()
{
    // Mark all the vertices as not visited and not part of recursion
    // stack
    bool *visited = new bool[V];
    for (int i = 0; i < V; i++)
        visited[i] = false;

    // Call the recursive helper function to detect cycle in different
    // DFS trees
    for (int u = 0; u < V; u++)
        if (!visited[u]) // Don't recur for u if it is already visited
            if (isCyclicUtil(u, visited, -1))
                return true;

    return false;
}

// Driver program to test above functions
int main()
{
    Graph g1(5);
    g1.addEdge(1, 0);
    g1.addEdge(0, 2);
    g1.addEdge(2, 0);
    g1.addEdge(0, 3);
    g1.addEdge(3, 4);
    g1.isCyclic() ? cout << "Undirected Graph isn't a tree\n" : cout
            << "Undirected Graph is a tree\n";

    Graph g2(3);
    g2.addEdge(0, 1);
    g2.addEdge(1, 2);
    g2.isCyclic() ? cout << "Undirected Graph isn't a tree\n" : cout
            << "Undirected Graph is a tree\n";

    return 0;
}


Output:

Undirected Graph isn't a tree
Undirected Graph is a tree

------------------
(program exited with code: 0)
Press return to continue



More C++ Programs: