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
     / \
     \_/

Hint

Train of Thought

Code

//DFS
/**
 * 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);
    }

    public UndirectedGraphNode clone(UndirectedGraphNode node) {
        if (node == null) {
            return null;
        }

        if (map.containsKey(node.label)) {
            return map.get(node.label);
        }

        UndirectedGraphNode temp = new UndirectedGraphNode(node.label);
        map.put(temp.label, temp);

        for (UndirectedGraphNode neighbor : node.neighbors) {
            temp.neighbors.add(clone(neighbor));
        }

        return temp;
    }
}

//BFS
public class Solution {
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        if (node == null) return null;

        UndirectedGraphNode newNode = new UndirectedGraphNode(node.label); //new node for return
        HashMap<Integer, UndirectedGraphNode> map = new HashMap(); //store visited nodes

        map.put(newNode.label, newNode); //add first node to HashMap

        LinkedList<UndirectedGraphNode> queue = new LinkedList(); //to store **original** nodes need to be visited
        queue.add(node); //add first **original** node to queue

        while (!queue.isEmpty()) { //if more nodes need to be visited
            UndirectedGraphNode n = queue.pop(); //search first node in the queue
            for (UndirectedGraphNode neighbor : n.neighbors) {
                if (!map.containsKey(neighbor.label)) { //add to map and queue if this node hasn't been searched before
                    map.put(neighbor.label, new UndirectedGraphNode(neighbor.label));
                    queue.add(neighbor);
                }
                map.get(n.label).neighbors.add(map.get(neighbor.label)); //add neighbor to new created nodes
            }
        }

        return newNode;
    }
}    

Complexity