Que23.java

import java.io.*;
import java.util.*;
import java.util.Iterator;
import java.util.LinkedList;

public class Que23 {
    private int V;
    private LinkedList<Integer> adj[];

    Que23(int v)
    {
        V = v;
        adj = new LinkedList[v];
        for (int i=0; i<v; ++i)
            adj[i] = new LinkedList();
    }

    void addEdge(int v,int w)
    {
        adj[v].add(w);
        adj[w].add(v);
    }

    Boolean isCyclicUtil(int v, Boolean visited[], int parent)
    {

        visited[v] = true;
        Integer i;

        Iterator<Integer> it = adj[v].iterator();
        while (it.hasNext())
        {
            i = it.next();

            if (!visited[i])
            {
                if (isCyclicUtil(i, visited, v))
                    return true;
            }

            else if (i != parent)
                return true;
        }
        return false;
    }

    Boolean isTree()
    {
        Boolean visited[] = new Boolean[V];
        for (int i = 0; i < V; i++)
            visited[i] = false;

        if (isCyclicUtil(0, visited, -1))
            return false;

        for (int u = 0; u < V; u++)
            if (!visited[u])
                return false;

        return true;
    }

    public static void main(String args[])
    {
        Que23 g1 = new Que23(5);
        g1.addEdge(1, 0);
        g1.addEdge(0, 2);
        g1.addEdge(0, 3);
        g1.addEdge(3, 4);

        if (g1.isTree())
            System.out.println("Graph1 is a Tree");
        else
            System.out.println("Graph1 is not a Tree");

        Que23 g2 = new Que23(5);
        g2.addEdge(1, 0);
        g2.addEdge(0, 2);
        g2.addEdge(2, 1);
        g2.addEdge(0, 3);
        g2.addEdge(3, 4);

        if (g2.isTree())
            System.out.println("Graph2 is a Tree");
        else
            System.out.println("Graph2 is not a Tree");

    }
}