合并排序链接列表

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

def mergeTwoLists(l1: ListNode, l2: ListNode) -> ListNode:
    if not l1:
        return l2
    if not l2:
        return l1
    if l1.val <= l2.val:
        l1.next = mergeTwoLists(l1.next, l2)
        return l1
    else:
        l2.next = mergeTwoLists(l1, l2.next)
        return l2