Clone Graph

https://oj.leetcode.com/problems/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 UndirectedGraphNode cloneGraph(UndirectedGraphNode node)

这一题虽然说是图的克隆，但其实本质上就是无向图的全遍历。那么DFS和BFS在这里都是可以用的。就是用HashMap来进行路径记录避免循环路径。下面直接给出两段代码吧

DFS（实际上是一个bottom-up回收路径的过程）：

    public UndirectedGraphNode DFShelper(UndirectedGraphNode node, HashMap<UndirectedGraphNode, UndirectedGraphNode> visited){
        if(visited.containsKey(node))
            return visited.get(node);
        UndirectedGraphNode copy_node = new UndirectedGraphNode(node.label);
        visited.put(node, copy_node);
        for(UndirectedGraphNode n : node.neighbors){
            copy_node.neighbors.add(DFShelper(n, visited));
        }
        return copy_node;
    }

BFS：

    public UndirectedGraphNode BFShelper(UndirectedGraphNode node){
        Queue<UndirectedGraphNode> node_queue = new LinkedList<UndirectedGraphNode>();
        HashMap<UndirectedGraphNode, UndirectedGraphNode> visited = new HashMap<UndirectedGraphNode, UndirectedGraphNode>();
        UndirectedGraphNode res = new UndirectedGraphNode(node.label);
        visited.put(node, res);
        node_queue.add(node);
        while(!node_queue.isEmpty()){
            UndirectedGraphNode cur_node = node_queue.poll();
            UndirectedGraphNode copy_node = visited.get(cur_node);
            for(UndirectedGraphNode n : cur_node.neighbors){
                UndirectedGraphNode copy_n = visited.containsKey(n) ? visited.get(n) : new UndirectedGraphNode(n.label);
                copy_node.neighbors.add(copy_n);
                if(!visited.containsKey(n))node_queue.add(n);
                visited.put(n, copy_n);
            }
        }
        return res;
        
    }

具体说一下哈希表的内容，键是原图的内容，值是拷贝的内容。所以每次DFS递归或者BFS循环的时候，总能在对应的点上取到对应的neighbors copy。BFS比较好理解，DFS实际上就是一个首先top-down遍历路径然后再bottom-up回收路径的一个做法。这在DFS上应该也是很常见的。