leetcode 133. Clone Graph（图的深复制）

Given a reference of a node in a connected undirected graph, return a deep copy (clone) of the graph. Each node in the graph contains a val (int) and a list (List[Node]) of its neighbors.

给出一个无向图，让做一个deep copy

deep copy就是不是仅仅copy reference，而是重新建立一个无向图

思路：
DFS遍历节点，其中为了防止重复遍历，把遍历过节点保存在HashMap, 当节点已经存在在HashMap中就说明已经访问过了，不需要再遍历。
而且在map中用旧的节点指向copy出的新的节点，一一对应关系

遍历完之后，再建立neighbors的关系
因为所有的旧节点和新节点已经有了一一对应的关系，把HashMap中的Key，也就是旧节点遍历一遍，取每个节点的neighbors，找到neighbors中每个Node对应的新节点，添加到新的neighbors

/*
class Node {
    public int val;
    public List neighbors;

    public Node() {}

    public Node(int _val,List _neighbors) {
        val = _val;
        neighbors = _neighbors;
    }
};
*/
class Solution {
    public Node cloneGraph(Node node) {
        Queue<Node> queue = new LinkedList<>();
        HashMap<Node, Node> map = new HashMap<Node, Node>();
        
        queue.offer(node);
        
        while (!queue.isEmpty()) {
            Node tmp = queue.poll();
            
            Node newNode = new Node(tmp.val, null);
            map.put(tmp, newNode);
            for (int i = 0; i < tmp.neighbors.size(); i++) {
                if (!map.containsKey(tmp.neighbors.get(i))) {
                    queue.offer(tmp.neighbors.get(i));
                    map.put(tmp.neighbors.get(i), null);
                }
            }           
        }
        
        for(Map.Entry<Node, Node> entry : map.entrySet()){
            Node current = entry.getKey();
            List<Node> neighbors = new ArrayList<Node>();
            for(Node tmpNode : current.neighbors) {
                 neighbors.add(map.get(tmpNode));
            }
            map.get(current).neighbors = neighbors;
        }
        
        return map.get(node);
    }
}