public class Graph {
    // instance variables go here:
    private boolean isDirected;
    private Vertex [] vertices;

    // constructor: n is the number of vertices, 
    // directed is a boolean which is true if the graph is directed,
    // false otherwise
    public Graph(int n, boolean directed) {
	    isDirected = directed;
	    vertices = new Vertex[n];
    }

    // startPoints are the starting points of edges, endPoints are the
    // ending points
    public void setEdges(int [] startPoints, int [] endPoints) 
	throws GraphException {
	
	if (startPoints.length != endPoints.length) 
	    throw new GraphException("invalid egdes data");

	// adding edges to the graph
	for (int i = 0; i < startPoints.length; ++i) {

	    // checking if vertices with these numbers exist
	    if (startPoints[i] >= vertices.length) 
		throw new GraphException("invalid starting vertex");
	    if (endPoints[i] >= vertices.length) 
		throw new GraphException("invalid ending vertex");

	    // adding the edge
	    vertices[startPoints[i]].addEdge(endPoints[i]);  

	    // adding back edge for undirected graph
	    if (!isDirected) {
		vertices[endPoints[i]].addEdge(startPoints[i]);
	    }
	}
    }

    public String toString() {
        return "";
    }
}

public class Vertex {
    private Edge edges;
    
    public void addEdge(int endPoint) {
	Edge edge = new Edge(endPoint);
	edge.setNext(edges);
	edges = edge;
    }
}

public class Edge {
    private int endPoint;
    private Edge next;

    public Edge(int endPoint) {
	this.endPoint = endPoint;
    }
    
    public void setNext(Edge edge) {
	next = edge;
    }
    
    public Edge getNext() {
	return next;
    }
    
    public int getEndPoint() {
	return endPoint;
    }
}

public class GraphException extends Exception {

    public GraphException(String s) {
	super(s);
    }
}

public class TestGraphs {
    public static void main(String [] args) {
	Graph g = new Graph(10,false); // undirected graph
	int [] starts = {0, 6, 3, 4, 2, 3, 5, 7, 8, 0, 9, 1};
	int [] ends = {1, 2, 0, 7, 3, 1, 2, 5, 6, 8, 4, 9};
	g.setEdges(starts, ends);
	System.out.println(g);
    }
}