LeetCode--No.138--Copy List with Random Pointer

A linked list is given such that each node contains an additional random pointer which could point to any node in the list or null.

Return a deep copy of the list.

Example 1:

Input:
{"$id":"1","next":{"$id":"2","next":null,"random":{"$ref":"2"},"val":2},"random":{"$ref":"2"},"val":1}

Explanation:
Node 1's value is 1, both of its next and random pointer points to Node 2.
Node 2's value is 2, its next pointer points to null and its random pointer points to itself.

Note:

1. You must return the copy of the given head as a reference to the cloned list.

/*
// Definition for a Node.
class Node {
    public int val;
    public Node next;
    public Node random;

    public Node() {}

    public Node(int _val,Node _next,Node _random) {
        val = _val;
        next = _next;
        random = _random;
    }
};
*/
class Solution {
    
    public Node copyRandomList(Node head) {
        if (head == null)   return null;
        if (map.containsKey(head)){
            return map.get(head);
        }
        
        Node node = new Node(head.val, null, null);
        map.put(head, node);
        node.next = copyRandomList(head.next);
        node.random = copyRandomList(head.random);
        return node;
    }
}

一开始没有想到递归. 以后拿到题还是要先想一下，递归能不能做，dp能不能做，然后再去想常规解法。
非递归的解法也没有看，今天好累，就不看了。以后有机会再见的话。

最近不是很开心，希望早日度过这两个月。一个人地铁，一个人吃饭，一个人刷题，也有点习惯了。至少不会无聊。
以后想要坚持每周六参加那个leetcode的竞赛。加油~！