Clone Graph

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

How we serialize an undirected graph:

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 #.

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
     / \
     \_/

Return a deep copy of the graph.

Solution:

Two steps:

• Copy each node and store the mapping information between original node and new created node in a hashmap.
• Copy each edge node by node according to mapping information that stored in hashmap.

Code:

public class Solution {

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

        HashMap<UndirectedGraphNode, UndirectedGraphNode> res = cloneNodes(node);
        copyNodeEdges(res);
        return res.get(node);
    }

    private HashMap<UndirectedGraphNode, UndirectedGraphNode> cloneNodes(
                    UndirectedGraphNode node) {
        Queue<UndirectedGraphNode> q = new LinkedList<>();
        HashSet<UndirectedGraphNode> visited = new HashSet<>();
        HashMap<UndirectedGraphNode, UndirectedGraphNode> res = new HashMap<>();

        q.offer(node);
        res.put(node, new UndirectedGraphNode(node.label));
        while (!q.isEmpty()) {
            UndirectedGraphNode temp = q.poll();
            res.put(temp, new UndirectedGraphNode(temp.label));
            for (UndirectedGraphNode x : temp.neighbors) {
                if (!visited.contains(x)) {
                    q.offer(x);
                    visited.add(x);
                }
            }
        }
        return res;
    }

    private void copyNodeEdges( 
                    HashMap<UndirectedGraphNode, UndirectedGraphNode> nodes) {
        Iterator it = nodes.entrySet().iterator();
        while (it.hasNext()) {
            Map.Entry<UndirectedGraphNode, UndirectedGraphNode> pair = 
                            (Map.Entry) it.next();
            for (UndirectedGraphNode x : pair.getKey().neighbors) {
                pair.getValue().neighbors.add(nodes.get(x));
            }
        }
    }
}