Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

这题与 Two Numbers相似，注意的是存在长度不等，以及考虑使用dummy node作为结果的头元素，时间复杂度O(n＋m）,n为两条链表分别的长度。空间复杂度O(1).代码如下：

class Solution(object):    def mergeTwoLists(self, l1, l2):        """        :type l1: ListNode        :type l2: ListNode        :rtype: ListNode        """        if not l1 and not l2:            return None        dummy = ListNode(-1)        cur = dummy        while l1 and l2:            if l1.val <  l2.val:                cur.next = l1                l1 = l1.next            else:                cur.next = l2                l2 = l2.next            cur = cur.next        cur.next = l1 if l1 else l2        return dummy.next