Clone Graph

题目：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
         / \
         \_/

解析：
    这道题主要是数据结构的应用吧，用一个队列存放图中还未被访问的结点，用一个map存放克隆的新结点。对于队列中的结点，访问其邻居，如果该邻居已经被克隆，那么只需维护克隆结点之间的邻居关系即可，如果该邻居尚未被克隆，那么需要new一个新的结点作为它的克隆，并维护克隆后的邻居关系。（BFS）

实现：

/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     List<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */
public class Solution {
    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;
    }
}