133- Clone Graph
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Total Accepted: 69310 Total Submissions: 278633 Difficulty: Medium

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.

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/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     List<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };

Visually, the graph looks like the following:
       1
      / \
     /   \
    0 --- 2
         / \
         \_/

 */
public class Solution {
    private HashMap<Integer, UndirectedGraphNode> map = new HashMap<>();
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        // https://leetcode.com/discuss/26988/depth-first-simple-java-solution
        if (node == null) return null;
        
        if (map.containsKey(node.label)) {
           return map.get(node.label);
        } 
        
        UndirectedGraphNode clone = new UndirectedGraphNode(node.label);
        map.put(node.label, clone);
        
        for (UndirectedGraphNode neighbor : node.neighbors) {
            clone.neighbors.add(cloneGraph(neighbor)); // dfs
        }
        
        return clone;
    }
}