19. 删除链表的倒数第 N 个结点

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def removeNthFromEnd(self, head: Optional[ListNode], n: int) -> Optional[ListNode]:
        dummy =ListNode(next =head )
        left =dummy
        right =head 
        while  n and right : 
            right=right.next 
            n -=1
        while right:
            left=left.next 
            right=right.next 
        left.next =left.next.next 
        return dummy.next