Solution.js

const mergeTwoLists2 = function (l1, l2) {
  if (l1 === null && l2 === null) {
    return null;
  }
  if (l1 !== null && l2 === null) {
    return l1;
  }
  if (l1 === null && l2 !== null) {
    return l2;
  }
  if (l1 !== null && l2 !== null) {
    let t = null, h = null;
    if (l1.val > l2.val) {
      t = l2;
      h = l2;
      l2 = l2.next;
    } else {
      t = l1;
      h = l1;
      l1 = l1.next;
    }
    while (l1 !== null && l2 !== null) {
      if (l1.val > l2.val) {
        t.next = l2;
        t = t.next;
        l2 = l2.next;
      } else {
        t.next = l1;
        t = t.next;
        l1 = l1.next;
      }
    }
    while (l1 !== null) {
      t.next = l1;
      l1 = l1.next;
      t = t.next;
    }
    while (l2 !== null) {
      t.next = l2;
      l2 = l2.next;
      t = t.next;
    }
    return h;
  }
}

const mergeTwoLists = function (l1, l2) {
  if (l1 === null) return l2;
  if (l2 === null) return l1;
  if (l1.val < l2.val) {
    l1.next = mergeTwoLists(l1.next, l2);
    return l1;
  } else {
    l2.next = mergeTwoLists(l1, l2.next);
    return l2;
  }
}