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
         / \
         \_/

一刷
题解：用DFS

/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     List<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */
public class Solution {
    private Map<Integer, UndirectedGraphNode> map = new HashMap<>();
    
    
    public UndirectedGraphNode cloneGraph(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(cloneGraph(neighbor));
        }
        return clone;
    }
    
}

二刷
注意，由于图内存在环，所以不能直接用DFS或BFS，而要用map排除掉已经进入构造或已经构造好的点（保存指针即可）

/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     List<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */
public class Solution {
    Map<Integer, UndirectedGraphNode> map = new HashMap<>();
    
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        if(node == null) return null;
        if(map.containsKey(node.label)) return map.get(node.label);
        UndirectedGraphNode cur = new UndirectedGraphNode(node.label);
        cur.neighbors = new ArrayList<>();
        map.put(node.label, cur);
        for(UndirectedGraphNode neigh : node.neighbors){
            cur.neighbors.add(cloneGraph(neigh));
        }
        return cur;
    }
}