Clone Graph

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
         / \
         \_/

Use BFS to copy each visited node and its neighbors, the key is to use a map to track already copied node in the graph.

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

        //BFS
        Queue<UndirectedGraphNode> queue = new LinkedList<UndirectedGraphNode>();    
        //map is used to store original map -> copied map
        Map<UndirectedGraphNode, UndirectedGraphNode> copiedMap = new HashMap<UndirectedGraphNode, UndirectedGraphNode>();

        UndirectedGraphNode newNode = new UndirectedGraphNode(node.label);
        queue.add(node);
        copiedMap.put(node, newNode);

        while (!queue.isEmpty()) {
            UndirectedGraphNode current = queue.poll();
            List<UndirectedGraphNode> neighbors = current.neighbors;
            for (UndirectedGraphNode neighbor : neighbors) {
                UndirectedGraphNode copiedNode = copiedMap.get(neighbor);
                //never visited before
                if (copiedNode==null) {
                    copiedNode = new UndirectedGraphNode(neighbor.label);
                    //map the new node with original node
                    copiedMap.put(neighbor, copiedNode);
                    //enqueue the neighbor
                    queue.add(neighbor);                                      
                } 
                //either a loop or a different node, copy all neighbors
                copiedMap.get(current).neighbors.add(copiedNode);
            }
        }

        return newNode;
    }