Clone Graph：无向有环图复制

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 #.

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
         / \
         \_/

思路：注意，此道题目输入是有环的，所以对于是否需要创建新的节点，是需要判断的，否则会出现一个节点被多次创建的情况。

采用深度优先遍历，如果节点没被创建过，则创建新节点，否则链接已创建的节点。

/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     List<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */
public class Solution {
    HashMap<Integer,UndirectedGraphNode> built = new HashMap<Integer,UndirectedGraphNode>();  
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        if(node == null ) return null;        
        UndirectedGraphNode root = new UndirectedGraphNode(node.label);
        built.put(root.label,root);
        for(UndirectedGraphNode neighbor : node.neighbors){
            if(!built.keySet().contains(neighbor.label)){
                UndirectedGraphNode n = cloneGraph(neighbor);
                if(n != null ){
                    root.neighbors.add(n);
                    built.put(n.label,n);
                }
            }else{
                UndirectedGraphNode temp = built.get(neighbor.label);
                root.neighbors.add(temp);
            }           
        }
        return root;
    }
}