138. Copy List with Random Pointer（js）

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.
题意：给定一个链表，每个节点包含一个额外增加的随机指针，该指针可以指向链表中的任何节点或空节点
代码如下：

/**
 * // Definition for a Node.
 * function Node(val,next,random) {
 *    this.val = val;
 *    this.next = next;
 *    this.random = random;
 * };
 */
/**
 * @param {Node} head
 * @return {Node}
 */
var copyRandomList = function(head) {
   const dummy = new Node(NaN, null, null); 
    let original = head; 
    let copier = dummy; 

    const map = new Map();
    while(original) {
        let newNode = new Node(original.val, null, null);
        copier.next = newNode;
        map.set(original, newNode);
        
        copier = copier.next;
        original = original.next;
    }
    copier = dummy.next;
    original = head;
    while(original) {
        if (original.random) {
            copier.random = map.get(original.random)
        }
        copier = copier.next
        original = original.next
    }
    
    return dummy.next
};