【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
         / 
         \_/

---

题解：利用DFS递归的方法克隆图，用hashmap保存原图中的节点和新图中的节点的对应关系。而且利用这个hashmap可以在O(1)的时间内判断一个节点是否被克隆过，并且知道克隆它得到的新节点。

实现一个克隆节点的方法，在这个方法中：

1. 判断这个节点是否被clone过了，如果是，返回它对应的clone节点；
2. 如果没有被clone过，那么新建节点为该节点的clone节点，并且递归的clone它的邻居节点，放入该节点的neighbors列表里面；

代码如下：

/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     List<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */
public class Solution {
    private HashMap<UndirectedGraphNode,UndirectedGraphNode> mapClone = new HashMap<UndirectedGraphNode,UndirectedGraphNode>();
    
    private UndirectedGraphNode cloneNode(UndirectedGraphNode node){
        if(node == null)
            return null;
        //if this node already has a clone
        if(mapClone.containsKey(node))
            return mapClone.get(node);
        
        //if this node hasn't been cloned, we construct a new node copy
        UndirectedGraphNode copy = new UndirectedGraphNode(node.label);
        mapClone.put(node, copy);
        
        //clone all the neighbers of node
        for(UndirectedGraphNode n:node.neighbors){
            copy.neighbors.add(cloneNode(n));
        }
        return copy;
    }
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        return cloneNode(node);
    }
}