19. 删除链表的倒数第 N 个结点

给你一个链表，删除链表的倒数第 n 个结点，并且返回链表的头结点。

示例 1：

输入：head = [1,2,3,4,5], n = 2
输出：[1,2,3,5]
示例 2：

输入：head = [1], n = 1
输出：[]
示例 3：

输入：head = [1,2], n = 1
输出：[1]

提示：

链表中结点的数目为 sz
1 <= sz <= 30
0 <= Node.val <= 100
1 <= n <= sz

class ListNode():
    def __init__(self, val=1,next=None):
        self.val=val
        self.next=next

class Solution():
    def findFromEnd(self, head: ListNode, k: int) -> ListNode:
        p1 = head
        for i in range(k):
            p1=p1.next
        p2 = head
        while p1 and p2:
            p1=p1.next
            p2=p2.next
        return p2
    
    def removeNthFromEnd(self, head: ListNode, n: int) -> ListNode:
        dummpy = ListNode(-1)
        dummpy.next = head
        p1 = self.findFromEnd(dummpy,n+1)
        p1.next = p1.next.next
        return dummpy.next