21. Merge Two Sorted Lists刷题笔记

用虚拟头节点，在处理的时候会方便许多

# 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]:
        if not list1:
            return list2
        head = pre = ListNode(val=0,next=list1)
        cur1 = list1
        cur2 = list2
        while cur1 and cur2:
            if cur1.val <= cur2.val:
                pre.next = cur1
                cur1 = cur1.next
            else:
                pre.next = cur2
                cur2 = cur2.next
            pre = pre.next
        if cur1:
            pre.next = cur1
        else:
            pre.next = cur2
        return head.next