Clone an undirected graph. Each node in the graph contains alabeland a list of itsneighbors.
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#.

1. First node is labeled as0. Connect node0to both nodes1and2.
2. Second node is labeled as1. Connect node1to node2.
3. Third node is labeled as2. Connect node2to node2(itself), thus forming a self-cycle.

Visually, the graph looks like the following:

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

/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     ArrayList<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */
import java.util.*;
public class Solution {
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        if(node == null) {
            return null;
        }
        Map<UndirectedGraphNode,UndirectedGraphNode> map = new HashMap();
        return clone(node,map);
    }
    public UndirectedGraphNode clone(UndirectedGraphNode node,Map<UndirectedGraphNode,UndirectedGraphNode> map) {
        if(node == null) {
            return null;
        }
        if(map.containsKey(node)) {
            return map.get(node);
        }
        UndirectedGraphNode newNode = new UndirectedGraphNode(node.label);
        map.put(node, newNode);
        for(UndirectedGraphNode index:node.neighbors) {
            newNode.neighbors.add(clone(index,map));
        }
        return newNode;
    }
}