# 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。 
# 
#  
# 
#  示例 1： 
# 
#  
# 输入：l1 = [1,2,4], l2 = [1,3,4]
# 输出：[1,1,2,3,4,4]
#  

方法一：递归，将较小值得节点参与到下一次比较中

# leetcode submit region begin(Prohibit modification and deletion)
# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
        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
# leetcode submit region end(Prohibit modification and deletion)