题目描述：
Clone an undirected graph. Each node in the graph contains a label and a list of its neighbors.
就是对一个图进行复制。
思路：
图遍历可以DFS或BFS，两种方式都可以，下面的解法是DFS的递归方式。当然，也可以用队列+哈希表使用BFS完成。
1. 在哈希表hash添加图的首节点source，和复制节点cloned（拷贝label）。
2. 复制过程：
2.1 如果source有neighbors，遍历neighbors，对于每个neighbors[i],判断是否在哈希存在，如果不存在，添加一份neighbor[i]的复制到哈希表中，进入递归；
2.2 如果已经存在，或者完成了2.1的复制步骤，将这个neighbor复制到当前map[source]对应的邻居列表中。即map[source].neighbors.Add(map[neighbor[i]])
实现代码：
/**
* Definition for undirected graph.
* public class UndirectedGraphNode {
* public int label;
* public IList<UndirectedGraphNode> neighbors;
* public UndirectedGraphNode(int x) { label = x; neighbors = new List<UndirectedGraphNode>(); }
* };
*/
public class Solution {
public UndirectedGraphNode CloneGraph(UndirectedGraphNode node)
{
if(node == null){
return null;
}
// use a map saving source - cloned node
var map = new Dictionary<UndirectedGraphNode, UndirectedGraphNode>();
// put root of source - cloned node into map
var cloned = new UndirectedGraphNode(node.label);
map.Add(node, cloned);
Clone(node, ref map);
return cloned;
}
private void Clone(UndirectedGraphNode source, ref Dictionary<UndirectedGraphNode, UndirectedGraphNode> map)
{
for(var i = 0;i < source.neighbors.Count; i++){
var n = source.neighbors[i];
if(!map.ContainsKey(n))
{
map.Add(n, new UndirectedGraphNode(n.label));
Clone(n, ref map);
}
map[source].neighbors.Add(map[n]);
}
}
}