[Leetcode] 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 #.

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
         / \
         \_/

The key idea is to use a HashMap to record the mapping relationship of the original node and copied node. We start from on unvisited node, make a copy of it, and insert this mapping relation into the hashmap. Then we start to copy the neighbors, if we find a node that is not contained in the hashmap, then this indicates that we have another unvisited node, we recurse on this node. After the recursion, the node has had a copy, then we could add this copy to the neighbors. Be careful when the input is null since hashmap does not allow null to be inserted and this would be a corner case.

    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        if(node == null)
            return null; // corner case
        HashMap<UndirectedGraphNode, UndirectedGraphNode> map = new HashMap<UndirectedGraphNode, UndirectedGraphNode>();
        dfs(node, map);
        return map.get(node);
    }
    
    private void dfs(UndirectedGraphNode node, HashMap<UndirectedGraphNode, UndirectedGraphNode> map) {
        UndirectedGraphNode newNode = new UndirectedGraphNode(node.label);
        map.put(node, newNode); // mark as visited
        for(UndirectedGraphNode un : node.neighbors) { // copy neighbors 
            if(map.containsKey(un) == false) // if neighbor not visited, there is no copyed node, recurse on this node
                dfs(un, map);
            newNode.neighbors.add(map.get(un));
        }
    }