// Stephanie's class notes.
// sorely missing comments and docstrings. sorry.
// Graph implemented with adjacency set
// Note: I have implemented the graph as a digraph. If you want to connection
// node A to node B, it puts in an edge from A to B, but not the corresponding
// edge from B to A.
import java.util.*;
public class Graph3 {
    private HashSet<GraphNode> nodes;
    
    public static class GraphNode {
        private HashSet<GraphNode> neighbors;
        String label;
        
        public GraphNode() {
            this.neighbors = new HashSet<GraphNode>();
            this.label = "";
        }
        
        public GraphNode(String label) {
            this.neighbors = new HashSet<GraphNode>();
            this.label = label;
        }
        
        public void connectTo( GraphNode other ) {
            this.neighbors.add( other );
        }           
        
        public String toString() { 
            String ret = this.label + ":";
            for (GraphNode n: this.neighbors)
                ret += " " + n.label;
            return ret;
        } 
    } // end class GraphNode
    
    public Graph3() {
        nodes = new HashSet<GraphNode>();   
    }
    
    public void connect( GraphNode fromNode, GraphNode toNode ) {
        if (! this.nodes.contains( fromNode ))
            this.nodes.add( fromNode );
        if (! this.nodes.contains( toNode ))
            this.nodes.add( toNode );
        fromNode.connectTo( toNode );
    }
    
    public void printAll() {
        for (GraphNode n: nodes)
            System.out.println( n );
    }
    
    public static void main( String[] args ) {
        Graph3 g = new Graph3();
        GraphNode n1 = new GraphNode( "n1" );
        GraphNode n2 = new GraphNode( "n2" );
        GraphNode n3 = new GraphNode( "n3" );
        g.connect( n1, n2 );
        g.connect( n3, n2 );
        g.printAll();
    }
}