Question:

Clone an undirected graph. Each node in the graph contains a label and a list of its neighbors.

OJ’s undirected graph serialization:
Nodes are labeled uniquely.

We use # as a separator for each node, and , as a separator for node label and each neighbor of the node.
As an example, consider the serialized graph {0,1,2#1,2#2,2}.

The graph has a total of three nodes, and therefore contains three parts as separated by #.

First node is labeled as 0. Connect node 0 to both nodes 1 and 2.
Second node is labeled as 1. Connect node 1 to node 2.
Third node is labeled as 2. Connect node 2 to node 2 (itself), thus forming a self-cycle.
Visually, the graph looks like the following:

   1
  / \
 /   \
0 --- 2
     / \
     \_/

Thinking:

In order to clone this graph, we need to traverse the graph either using DFS or BFS. In my solution, I use DFS and graph search which will hold a HashMap to record the explored nodes.

Solution:

HashMap<Integer, UndirectedGraphNode> exploredSet = new HashMap<Integer, UndirectedGraphNode>();
public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
    return clone(node);
}

private UndirectedGraphNode clone(UndirectedGraphNode node){
    if (node == null) return null;
    UndirectedGraphNode newNode = new UndirectedGraphNode(node.label);

    exploredSet.put(node.label, newNode);

    for (UndirectedGraphNode n: node.neighbors) {
        if (exploredSet.containsKey(n.label)) {
            newNode.neighbors.add(exploredSet.get(n.label));
        } else {
            newNode.neighbors.add(clone(n));
        }
    }

    return newNode;
}