[leedcode 133] 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
         / 
         \_/

/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     List<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */
public class Solution {
    使用深度优先搜索。这样可以递归实现，如果是宽度优先，就要额外使用queue容器。
    关键点：
    1 这里的clone需要深度拷贝，就是要使用new操作了
    2 防止回路无限循环，就要使用hash表，这里使用map记录访问过的节点。因为这里的label应该是唯一的才对，所以可以直接使用label作为关键字就可以。
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        if(node==null) return null;
        HashMap<Integer,UndirectedGraphNode> map=new HashMap<Integer,UndirectedGraphNode>();
        return clone(node,map);
    }
    public UndirectedGraphNode clone(UndirectedGraphNode node,HashMap<Integer,UndirectedGraphNode> map){
        if(map.containsKey(node.label)) return map.get(node.label);
        UndirectedGraphNode newNode=new UndirectedGraphNode(node.label);
         map.put(node.label,newNode);////注意标记的位置
        for(UndirectedGraphNode n:node.neighbors){
            newNode.neighbors.add(clone(n,map));/////
        }     
        return newNode;
    }
}