leetcode 21 合并两个有序链表

https://www.bilibili.com/video/BV1xa411A76q?p=4&vd_source=cdfcf738e0429ec2cffe4778dd8dd0e4

 

# 迭代
# https://blog.csdn.net/m0_61661179/article/details/127205244
class Solution:
    def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
        preindex = prevalue = ListNode(-1)
        while list1 and list2:
            if list1.val <= list2.val:
                prevalue.next = list1
                list1 = list1.next
            else:
                prevalue.next = list2
                list2 = list2.next
            prevalue = prevalue.next
        # 合并后，剩余部分，直接指完
        prevalue.next = list1 if list1 is not None else list2
        return preindex.next