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
         / \
         \_/

Java代码：

class UndirectedGraphNode {
    int label;
    List<UndirectedGraphNode> neighbors;
    UndirectedGraphNode(int x) {
        label = x;
        neighbors = new ArrayList<UndirectedGraphNode>();
    }
}

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

    UndirectedGraphNode clone = new UndirectedGraphNode(node.label);

    //用一个Map来保存已经克隆的新结点
    HashMap<Integer, UndirectedGraphNode> map = new HashMap<Integer, UndirectedGraphNode>();
    map.put(node.label, clone);

    //用一个队列来保存没有访问过的结点
    LinkedList<UndirectedGraphNode> queue = new LinkedList<UndirectedGraphNode>();
    queue.add(node);

    while (!queue.isEmpty()) {
        UndirectedGraphNode originalnode = queue.remove();
        UndirectedGraphNode clonenode = map.get(originalnode.label);

        for (int i = 0; i < originalnode.neighbors.size(); i++) {
            UndirectedGraphNode neighbor = originalnode.neighbors.get(i);

            //如果该邻居结点已被克隆
            if (map.get(neighbor.label) != null) {
                clonenode.neighbors.add(map.get(neighbor.label));
                continue;
            }

            //把该邻居放入queue中
            queue.add(neighbor);

            //克隆该邻居结点
            UndirectedGraphNode newnode = new UndirectedGraphNode(neighbor.label);

            //把新结点放入map中
            map.put(neighbor.label, newnode);

            //把新结点加入邻居集中
            clonenode.neighbors.add(newnode);
        }
    }

    return clone;
}