Clone Graph

/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     ArrayList<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */
public class Solution {
    // key: original node
    // value: copied node
    HashMap<UndirectedGraphNode, UndirectedGraphNode> map = new HashMap<UndirectedGraphNode, UndirectedGraphNode>();
    
    private UndirectedGraphNode cloneHelper(UndirectedGraphNode node) {
        if (node == null) {
            return null;
        }
        
        if (map.containsKey(node)) {
            return map.get(node);
        } else {
            UndirectedGraphNode copy = new UndirectedGraphNode(node.label);
            map.put(node, copy);
            
            for (UndirectedGraphNode n : node.neighbors) {
                copy.neighbors.add(cloneHelper(n));
            }
            
            return copy;
        }
    }
    
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        return cloneHelper(node);
    }
}