Merge Two Sorted Lists

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Add 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