LEETCODE 21. 合并两个有序链表

# 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, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
        dummy_node=ListNode(-1)
        cur=dummy_node
        l1=list1
        l2=list2
        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 dummy_node.next