Clone Graph

Clone an undirected graph. Each node in the graph contains a label and a list of its neighbours.
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 neighbour 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 as 0. Connect node 0 to both nodes 1 and 2.
2. Second node is labeled as 1. Connect node 1 to node 2.
3. 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
        / \
        \_/

分析