leetcode--18. clone-graph

题目：
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 #.
First node is labeled as 0. Connect node0 to both nodes 1 and 2.
Second node is labeled as 1. Connect node1 to node2.
Third node is labeled as 2. Connect node2 to node2(itself), thus forming a self-cycle.

克隆无向图。图中的每个节点都包含一个邻域列表。
OJ的无向图序列化：
节点是唯一标记的。我们使用 # 作为每个节点的分隔符，并用作节点标签和节点的每个邻居的分隔符。例如，序列化图{0，1，2 # 1，2 # 2，2}。
该图共有三个节点，因此包含三个部分，由#分隔。
第一个节点被标记为0。将节点0连接到节点1和2。
第二个节点被标记为1。将节点1连接到节点2。
第三个节点被标记为2。将节点2连接到节点2(本身)，从而形成一个自循环。

思路：
考察图的遍历算法， DFS / BFS。
基本思想， 利用一个map数据结构， 在遍历图的过程中， 创建相对应的新的图的部分

import java.util.*;
public class Solution {
    private HashMap<Integer, UndirectedGraphNode> map = new HashMap<>();
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        return clone(node);
    }
    
    private UndirectedGraphNode clone(UndirectedGraphNode node) {
        if (node == null) 
            return null;
        if (map.containsKey(node.label))
            return map.get(node.label);
        
        UndirectedGraphNode clone = new UndirectedGraphNode(node.label);
        map.put(clone.label, clone);
        for (UndirectedGraphNode neighbor : node.neighbors)
            clone.neighbors.add(clone(neighbor));
        return clone;
    }
     
}