Question

I have a Java class, Node as follows :

class Node
{
    public ArrayList<Node> nbrs;
}

Each Node object contains a list of all its neighbours within the ArrayList nbrs, and nothing else.

Now I need to write a function :

public Node copy( Node curr )

This function should perform a deep copy of the entire graph rooted at curr, and return the equivalent copy for curr.

I tried implementing a copy constructor within the class Node as follows :

public Node( Node n )
{
    for( Node curr : n.nbrs )
        n.nbrs.add( new Node( curr  ));
}

I now copy the Node n, within my copy function.

But I have found that when the graph contains loops, this code keeps running infinitely.

Any help on how I should overcome this problem.

PS : This is an interview question faced by my friend, so the class Node cannot contain any more variables

Answer 1

The standard trick is to first create all the new nodes and store them in a map (from old nodes to new nodes). Then in a second pass over all the nodes, all the edges are added (by adding to n.nbrs.add).