1. var sortList = function (head) {
    2.     if (!head || !head.next)  return head;
    3.     let slow = head, fast = head;
    4.     let preSlow = null;
    5.     while (fast && fast.next) {
    6.         preSlow = slow;
    7.         slow = slow.next;
    8.         fast = fast.next.next;
    9.     }
    10.     preSlow.next = null;
    11.     const l = sortList(head);
    12.     const r = sortList(slow);
    13.     return merge(l, r);
    14. };
    16. function merge(l1, l2) {
    17.     const dummy = new ListNode(0);
    18.     let prev = dummy;
    19.     while (l1 && l2) {
    20.         if (l1.val < l2.val) {
    21.             prev.next = l1;
    22.             l1 = l1.next;
    23.         } else {
    24.             prev.next = l2;
    25.             l2 = l2.next;
    26.         }
    27.         prev = prev.next;
    28.     }
    29.     if (l1) prev.next = l1;
    30.     if (l2) prev.next = l2;
    31.     return dummy.next;
    32. }