021 merge two sorted lists
21. Merge Two Sorted Lists
题目：
https://leetcode.com/problems/merge-two-sorted-lists/
难度: Easy
同样适用dummy head
class Solution(object):
def mergeTwoLists(self, l1, l2):
"""
:type l1: ListNode
:type l2: ListNode
:rtype: ListNode
"""
if l1 == None:
return l2
if l2 == None:
return l1
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
if l1:
cur.next = l1
else:
cur.next = l2
return dummy.next
