Clone an undirected graph. Each node in the graph contains a label and a list of its neighbors.

Nodes are labeled uniquely.

As an example, consider the serialized graph {0

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
         /          \_/

/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     List<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 
 
*/
public class Solution {
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        if (node == null) {
            return null;
        }
        Map<UndirectedGraphNode, UndirectedGraphNode> map = new HashMap<>();
        Set<UndirectedGraphNode> visited = new HashSet<>();
        Queue<UndirectedGraphNode> toBeVisited = new LinkedList<>();
        
        toBeVisited.offer(node);
        
        while (!toBeVisited.isEmpty()) {
            UndirectedGraphNode current = toBeVisited.poll();
            if (visited.contains(current)) {
                continue;
            }
            if (map.get(current) == null) {
                map.put(current, new UndirectedGraphNode(current.label));
            }
            for (UndirectedGraphNode neighbor : current.neighbors) {
                if (map.get(neighbor) == null) {
                    map.put(neighbor, new UndirectedGraphNode(neighbor.label));
                }
                map.get(current).neighbors.add(map.get(neighbor));
                toBeVisited.offer(neighbor);
            }
            visited.add(current);
        }
        
        return map.get(node);
    }
}

[LeetCode] 133. Clone Graph