题目：

输入两个递增排序的链表，合并这两个链表并使新链表中的节点仍然是递增排序的。
示例1：
输入：1->2->4, 1->3->4
输出：1->1->2->3->4->4
限制：
0 <= 链表长度 <= 1000

题目的理解：

需要理解是递增的单向链表，并且需要考虑链表为空的情况。

python实现

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
        left = l1
        right = l2

        if left is None and right is None:
            return None
        elif left is None:
            return right
        elif right is None:
            return left
        else:
            if left.val > right.val:
                result = ListNode(right.val)
                right = right.next
            else:
                result = ListNode(left.val)
                left = left.next
                
        current = result

        while True:
            if left is None and right is None:
                break
            elif left is None:
                current.next = right
                break
            elif right is None:
                current.next = left
                break
            
            if left.val > right.val:
                current.next = ListNode(right.val)
                right = right.next
            else:
                current.next = ListNode(left.val)
                left = left.next
            
            current = current.next
            
        return result