递归的思想，我一看！哦！惊为天人。

    1. var mergeTwoLists = function(l1, l2) {
    2.     if (!l1) {
    3.         return l2
    4.     }
    5.     if (!l2) {
    6.         return l1
    7.     }
    8.     let res = l1.val > l2.val ? l2 : l1
    9.     res.next = mergeTwoLists(res.next, l1.val > l2.val ? l1 : l2)
    10.     return res
    11. };