https://leetcode.com/problems/flatten-a-multilevel-doubly-linked-list/

flattenMultilevelDoublyLinkedList.js

/**
 * // Definition for a Node.
 * function Node(val,prev,next,child) {
 *    this.val = val;
 *    this.prev = prev;
 *    this.next = next;
 *    this.child = child;
 * };
 */

/**
 * @param {Node} head
 * @return {Node}
 */
var flatten = function(head) {
    if(!head) return head;
    
    const dfs = (node) => {
        if(!node.child){
            const tail = dfs(node.child);
            tail.next = node.next;
            if(node.next) node.next.prev = tail;
            node.next = node.child;
            node.child.prev = node;
            node.child = null;
        }
        return node.next ? dfs(node.next) : node;
    };
    
    dfs(head);
    return head;
};