删除链表的倒数第N个节点

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def removeNthFromEnd(self, head, n):
        """
        :type head: ListNode
        :type n: int
        :rtype: ListNode
        """
        first = head
        length = 0
        while first:#求长度
            length += 1
            first = first.next
            
        if length == 1:#如果长度为1，则n等于1
            head = None
            return head
        
        if length == n:#如果长度和n相等，则删除的是第一个节点
            head = head.next
            return head
        
        flag = 1
        pre = head
        cur = head.next
        while (flag < length - n):
            flag += 1
            pre = cur
            cur = cur.next
        pre.next = cur.next
        return head