Leetcode: 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
         / \
         \_/

Solution:

/**
 * 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) {
        return cloneGraph(node, new HashMap<Integer,UndirectedGraphNode>());
    }
    
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node, HashMap<Integer,UndirectedGraphNode> map) 
    {
        if(node == null)
            return null;
        if (map.containsKey(node.label))
            return map.get(node.label);
        UndirectedGraphNode newNode = new UndirectedGraphNode(node.label);
        map.put(node.label, newNode);
        for(UndirectedGraphNode neighbor: node.neighbors)
            newNode.neighbors.add(cloneGraph(neighbor, map));
        return newNode;
    }
}