LeetCoder - 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;
 *     ArrayList<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */
public class Solution{ 
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        if(node==null) {
            return null;
        }
        HashMap<Integer, UndirectedGraphNode> origMap = new HashMap<Integer, UndirectedGraphNode>();
        visit(node, origMap);
        
        HashMap<Integer, UndirectedGraphNode> newMap = new HashMap<Integer, UndirectedGraphNode>();
        for(Integer label : origMap.keySet()) {
            UndirectedGraphNode newNode = new UndirectedGraphNode(label);
            newMap.put(label, newNode);
        }
        
        for(Integer label : origMap.keySet()) {
            UndirectedGraphNode oldNode = origMap.get(label);
            UndirectedGraphNode newNode = newMap.get(label);
            for(UndirectedGraphNode neighbor : oldNode.neighbors) {
                newNode.neighbors.add(newMap.get(neighbor.label));
            }
        }
        return newMap.get(node.label);
    }
    
    private void visit(UndirectedGraphNode node, HashMap<Integer, UndirectedGraphNode> map) {
        map.put(node.label, node);
        for(UndirectedGraphNode neighbor : node.neighbors) {
            if(!map.containsKey(neighbor.label)) {
                visit(neighbor, map);
            }
        }
    }
}