problems with eulerian cycle in a directed graph

Question

so below is my code for finding if a graph has a eulerian cycle in a directed graph. The code works for several case(the commented lines in my main method works). But it does work for the g1 graph(the uncommented code in my main method) . it says the the graph (g1) does not that an eulerian circuit, which is should.Please help me find out the bug, thanks

  import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.Stack;


public class EulerianCirlsDirected {

        int v;
        Set<Integer> visitedDFS;
        int in[];
        HashMap<Integer,List<Integer>> adjList;
        public EulerianCirlsDirected(int v){
            this.visitedDFS = new HashSet<Integer>();
            this.adjList = new HashMap<Integer,List<Integer>>();
            this.v = v;
            in = new int[v];
        }

        //add edge
        public void addEdge(int src, int dest){
            List<Integer> srcNeighbour = this.adjList.get(src);
            if(srcNeighbour == null){
                this.adjList.put(src, srcNeighbour = new ArrayList<Integer>());
            }
            srcNeighbour.add(dest);
            in[dest]++;
        }


        //get neighbours of vertex
        public Iterable<Integer> getNeighbours(Integer vertex){

            List<Integer> neighbours = this.adjList.get(vertex);
            if(neighbours == null){
                return Collections.emptyList();
            }
            else{
                return Collections.unmodifiableList(neighbours);
            }

        }

        public int sizeNeighbours(Integer vertex){

            List<Integer> list = (List<Integer>) this.getNeighbours(vertex);


            return list.size();
        }

        //depth first search
        public Iterable<Integer> DFS(Integer src){
            Stack<Integer> stack= new Stack<Integer>();
            List<Integer> paths = new ArrayList<Integer>();
            stack.add(src);
            visitedDFS.add(src);
            paths.add(src);

            while(!stack.isEmpty()){

                int ref = stack.pop();
                for(int neig : this.getNeighbours(ref)){
                    if(!visitedDFS.contains(neig)){
                        stack.add(neig);
                        visitedDFS.add(neig);
                        paths.add(neig);
                    }
                }


            }

            return Collections.unmodifiableSet(visitedDFS);
        }

        public int numVertices(){
            return this.v;
        }

        //transpose of graph
        public EulerianCirlsDirected getTranspose(){

            int v = this.numVertices();
            EulerianCirlsDirected gr = new EulerianCirlsDirected(v);
            for(int i=0; i<v;i++){
                for(int neig:this.getNeighbours(i)){
                    gr.addEdge(neig, i);
                }
            }
            return gr;

        }

        //checks if graph has a eulerian cycle
        public boolean isEulerianCycle(){
            int v = this.numVertices();

            //check if graph is connect
            //that is every non zero degree vertex is part 
            //of a strongly connected component
            if(!isConnected()){
                return false;
            }

            //check for indegree and out degree
            for(int i=0;i<v;i++){
                if(in[i] != this.adjList.get(i).size()){
                    return false;
                }
            }

            return true;

        }

        //method to verify for strongly connected component
        public boolean isConnected(){

            int v =this.numVertices();
            int i;

            //get the first non zero degre vertex
            for( i=0; i<v;i++){
                if(this.sizeNeighbours(i)>0){break;}
            }

            //first run dfs for original graph at the first non
            //zero degree vertex
            this.DFS(i);

            //check if all vertices where visited during dfs
            for(int j=0;j<v;j++){
                if(!visitedDFS.contains(j)){
                    System.out.println("first " +visitedDFS.contains(j));
                    return false;
                }

            }

            //get transpose of graph and run dfs
            //so we have to reset visitedDFS
            visitedDFS.clear();
            EulerianCirlsDirected gr = this.getTranspose();

            //update visitedDFS to be that of the 
            //transpose
            visitedDFS= (Set<Integer>) gr.DFS(i);

            //check again if all vertices are visited in the 
            //transposed graph

            int grV = gr.numVertices();
            for(int j=0;j<grV;j++){
                if(!visitedDFS.contains(j)){
                    return false;
                }

            }
            return true;

        }
        public static void main(String[]args){

    //          EulerianCirlsDirected g = new EulerianCirlsDirected(2);
    //          g.addEdge(0, 1);
    //          g.addEdge(1, 0);
    //          g.addEdge(2, 3);
    //          g.addEdge(3, 0);
    //          g.addEdge(2, 4);
    //          g.addEdge(4, 2);

            EulerianCirlsDirected g1 = new EulerianCirlsDirected(26);
            g1.addEdge(6, 10);
            g1.addEdge(10, 6);
            System.out.println(g1.isEulerianCycle());
            //System.out.println(g.sizeNeighbours(1));

        }

    }

the output is false. please help

Answer 1

When you construct EulerianCirlsDirected with 26, your code expects that there will be 26 vertices and that they will all be touched by DFS.

Replace

EulerianCirlsDirected g1 = new EulerianCirlsDirected(26);
g1.addEdge(6, 10);
g1.addEdge(10, 6);

with

EulerianCirlsDirected g1 = new EulerianCirlsDirected(2);
g1.addEdge(0, 1);
g1.addEdge(1, 0);

and it will work.

Alternatively, check for this.sizeNeighbours(i) > 0 every time you iterate from 0 to v. Twice in isConnected() and once in isEurlerianCycle()