# 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:
        head = ListNode(0)
        res = head
        while l1 and l2:
            if l1.val <= l2.val:
                head.next = l1
                l1 = l1.next
                head = head.next
            else:
                head.next = l2
                l2 = l2.next
                head = head.next
        if not l1 and l2:
            head.next = l2
        if not l2 and l1:
            head.next = l1
        return res.next

class Solution:
    def mergeTwoLists(self, l1, l2):
        prehead = ListNode(-1)

        prev = prehead
        while l1 and l2:
            if l1.val <= l2.val:
                prev.next = l1
                l1 = l1.next
            else:
                prev.next = l2
                l2 = l2.next            
            prev = prev.next

        # 合并后 l1 和 l2 最多只有一个还未被合并完，我们直接将链表末尾指向未合并完的链表即可
        prev.next = l1 if l1 is not None else l2

        return prehead.next