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
     / \
     \_/

Solution

/**
 * 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<Integer, UndirectedGraphNode> map = new HashMap<Integer, UndirectedGraphNode>();
        Set<Integer> visited = new HashSet<Integer>();
        Queue<UndirectedGraphNode> queue = new LinkedList<UndirectedGraphNode>();
        int rootLabel = node.label;
        visited.add(node.label); // mark the first node as visited
        queue.offer(node); // add the first node in
        while(!queue.isEmpty()) {
            UndirectedGraphNode curnode = queue.poll(); // get the real node
            if(!map.containsKey(curnode.label)) map.put(curnode.label, new UndirectedGraphNode(curnode.label));
            UndirectedGraphNode clonedNode = map.get(curnode.label); // get the cloned node
            for(UndirectedGraphNode neighbor: curnode.neighbors) {
                if(!map.containsKey(neighbor.label)) map.put(neighbor.label, new UndirectedGraphNode(neighbor.label));
                clonedNode.neighbors.add(map.get(neighbor.label)); // create relations in the cloned node
                if(visited.add(neighbor.label)) queue.offer(neighbor);
            }
        }
        return map.get(rootLabel);
    }
}