CloneGraph.java

/*
Given the head of a graph, return a deep copy (clone) of the graph. Each node in the graph contains a label (int) and a list (List[UndirectedGraphNode]) of its neighbors. There is an edge between the given node and each of the nodes in its neighbors.


OJ's undirected graph serialization (so you can understand error output):
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
         / \
         \_/
Note: The information about the tree serialization is only meant so that you can understand error output if you get a wrong answer. You don't need to understand the serialization to solve the problem.
*/

/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     List<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */
public class Solution {
    private HashMap<Integer, UndirectedGraphNode> map = new HashMap<>();
    
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) 
    {
        return clone(node);
    }

    private UndirectedGraphNode clone(UndirectedGraphNode node) 
    {
        if (node == null) 
        {
            return null;
        }
        
        if (map.containsKey(node.label)) 
        {
            return map.get(node.label);
        }
        
        UndirectedGraphNode clone = new UndirectedGraphNode(node.label);
        map.put(clone.label, clone);
        
        for (UndirectedGraphNode neighbor : node.neighbors) 
        {
            clone.neighbors.add(clone(neighbor));
        }
        
        return clone;
    }
}