leetcode_remove nth node from end of list

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

class :
    def removeNthFromEnd(self, head, n):
        """
        :type head: ListNode
        :type n: int
        :rtype: ListNode
        """
        p = head
        length = 0
        while p != None:
            p = p.next
            length += 1
        
        if n > length:
            return head
        elif n == length:
            return head.next
        else:
            p = head
            k = length - n - 1
            while k != 0:
                k -= 1
                p = p.next
            p.next = p.next.next
            return head