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         / \         \_/

简直了，写了那么久的DFS，现在竟然写不出来了，怎么了？？？

DFS不是有套路吗？

dfs() {if() ..... return;for(.....) {dfs()}}

向下面这样的DFS是不对的，

（1）DFS最开始是先要一直调用一直调用直到return，下面就不是（dfs一般出现在第一句）

（2）既然是递归，为什么会for循环里面和循环外面都有

UndirectedGraphNode root = new UndirectedGraphNode(node.label);

要么就在for循环里面，要么就把for循环里的这个语句通过递归转移到循环外面

public class Solution {Map<Integer, UndirectedGraphNode> map = new HashMap<Integer, UndirectedGraphNode>();    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {    if(node == null)return null;    if(map.containsKey(node.label))return map.get(node.label);        UndirectedGraphNode root = new UndirectedGraphNode(node.label);    map.put(node.label, new UndirectedGraphNode(node.label));        for(UndirectedGraphNode n : node.neighbors) {    if(!map.containsKey(n.label)) {    UndirectedGraphNode newNode = new UndirectedGraphNode(n.label);    map.put(n.label, newNode);    map.get(node.label).neighbors.add(map.get(n.label));    cloneGraph(n);    } else    map.get(node.label).neighbors.add(map.get(n.label));            }        return root;    }}

正确的是：

import java.util.HashMap;import java.util.Map;public class Solution {Map<Integer, UndirectedGraphNode> map = new HashMap<Integer, UndirectedGraphNode>();    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {    if(node == null)return null;    if(map.containsKey(node.label))return map.get(node.label);        UndirectedGraphNode root = new UndirectedGraphNode(node.label);    map.put(node.label, root);        for(int i=0; i<node.neighbors.size(); i++) {    UndirectedGraphNode childRoot = cloneGraph(node.neighbors.get(i));    root.neighbors.add(childRoot);    }        return root;    }}

先要看能不能用DFS，能用的话怎么用？

比如这里：先把neighbor给clone掉，再把返回值添加到ArrayList里面就好了，思路应该是比较清晰的，看了还是缺少总结啊