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

flatten_list_no_stack.js

/* stackless version */

var flatten = function(head) {
        const first = head;
        var stack = null;
        while (head != null) {
            if (head.child != null) {
                if (head.next != null) {
                    head.next.prev = stack;
                    stack = head.next;
                }
                head.next = head.child;
                head.next.prev = head;
                head.child = null;
            }
            if (head.next == null && stack != null) {
                head.next = stack;
                stack = stack.prev;
                head.next.prev = head;
            }
            head = head.next;
        }
        return first;
    };

flatten_list_stack.js

/* stack version */

var flatten = function(head) {
        const first = head;
        var stack = [];
        while (head != null) {
            if (head.child != null) {
                if (head.next != null) {
                    head.next.prev = null;
                    stack.push(head.next);
                }
                head.next = head.child;
                head.next.prev = head;
                head.child = null;
            }
            if (head.next == null && stack.length > 0) {
                head.next = stack.pop();
                head.next.prev = head;
            }
            head = head.next;
        }
        return first;
    };