LeetCode 133: Clone Graph

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
         / \
         \_/

/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     List<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */
public class Solution {
    // Build relation for <node, NewNode>
    private Map<UndirectedGraphNode, UndirectedGraphNode> map;

    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        // We can use dfs or bfs for the traversal of all of the nodes
        // Here we adopt dfs
        if (node == null)
            return null;
            
        map = new HashMap<>();
        UndirectedGraphNode newNode = new UndirectedGraphNode(node.label);
        map.put(node, newNode);
        
        dfs(node);
        
        return newNode;
    }
    
    private void dfs(UndirectedGraphNode node)
    {
        if (node == null)
            return;
        
        for (UndirectedGraphNode neighbor : node.neighbors)
        {
            if (!map.containsKey(neighbor))
            {
                UndirectedGraphNode newNeighbor = new UndirectedGraphNode(neighbor.label);
                map.put(neighbor, newNeighbor);
                dfs(neighbor);
            }
            
            // Build neighbor relationship for new nodes
            map.get(node).neighbors.add(map.get(neighbor));
        }
    }
}