将两个升序链表合并为一个新的 升序 链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。
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输入：l1 = [1,2,4], l2 = [1,3,4]
输出：[1,1,2,3,4,4]
示例 2：
输入：l1 = [], l2 = []
输出：[]
示例 3：
输入：l1 = [], l2 = [0]
输出：[0]
迭代法：
head = ListNode(0)
cur = head
while l1 and l2:
if l1.val <=l2.val:
cur.next = l1
l1= l1.next
cur =cur.next
else:
cur.next = l2
l2 = l2.next
cur =cur.next
if l1 :
cur.next = l1
if l2 :
cur.next = l2
return head.next
迭代法
if l1 is None:
return l2
elif l2 is None:
return l1
elif l1.val <= l2.val:
l1.next = self.mergeTwoLists(l1.next, l2)
return l1
else:
l2.next = self.mergeTwoLists(l1, l2.next)
return l2
