fix: leetcode75 code format

fix: leetcode75 code format

Author: hj.tian

leetcode75/day18_19.py

@@ -6,18 +6,19 @@
 class Solution:
  def removeNthFromEnd(self, head: Optional[ListNode], n: int) -> Optional[ListNode]:
  fast, slow, pre = head, head, head
- while n > 0:
+ while n > 0 and fast:
  fast = fast.next
  n -= 1
- while fast:
+ while fast and slow:
  fast = fast.next
  pre = slow
  slow = slow.next
- if pre == slow:
+ if pre and slow and pre == slow:
  head = slow.next
  slow.next = None
- pre.next = slow.next
- slow.next = None
+ if pre and slow:
+ pre.next = slow.next
+ slow.next = None
  return head