题目：
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 / \ \_/思路：
解决图的问题的最大神器依然是DFS和BFS。由于图有可能会形成回路，所以我们建立一个哈希表来存储已经clone后的节点。在本题目中，我们采用DFS的思路，代码十分简练和易懂。
代码：
/** * Definition for undirected graph. * struct UndirectedGraphNode { * int label; * vector<UndirectedGraphNode *> neighbors; * UndirectedGraphNode(int x) : label(x) {}; * }; */ class Solution { public: UndirectedGraphNode *cloneGraph(UndirectedGraphNode *node) { if(node == NULL) { return NULL; } if(hash.count(node)) { return hash[node]; } hash[node] = new UndirectedGraphNode(node->label); for(auto val : node->neighbors) { hash[node]->neighbors.push_back(cloneGraph(val)); } return hash[node]; } private: unordered_map<UndirectedGraphNode*, UndirectedGraphNode*> hash; };