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 #.

// First node is labeled as 0. Connect node 0 to both nodes 1 and 2.
// Second node is labeled as 1. Connect node 1 to node 2.
// 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 CloneGraph {
    public HashMap<Integer, UndirectedGraphNode> map = new HashMap<Integer, UndirectedGraphNode>();
 
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        if(node == null) {
            return null;
        }
     
        if(map.containsKey(node.label)) {
            return map.get(node.label);
        }
     
        UndirectedGraphNode newNode = new UndirectedGraphNode(node.label);
        map.put(newNode.label, newNode);
     
        for(UndirectedGraphNode neighbor : node.neighbors) {
            newNode.neighbors.add(cloneGraph(neighbor));
        }
     
        return newNode;
    }
}