# Definition for singly-linked list.

# class ListNode(object):

#     def __init__(self, x):

#         self.val = x

#         self.next = None

class Solution(object):

    def removeNthFromEnd(self, head, n):

        """

        :type head: ListNode

        :type n: int

        :rtype: ListNode

        """

        dummy = ListNode(-1)

        dummy.next = head

        slow = fast = dummy

        #print(n,head.val)

        while n:

            fast = fast.next

            n -= 1

        #print(n,head.val,fast)

        while fast and fast.next:

            fast = fast.next

            slow = slow.next

        slow.next = slow.next.next

        return dummy.next

# Definition for singly-linked list.

# class ListNode(object):

#     def __init__(self, x):

#         self.val = x

#         self.next = None

class Solution:

    def removeNthFromEnd(self, head, n):

        fast = slow = head

        for _ in range(n):

            fast = fast.next

        if not fast:   #例如1-》2-》3-》4-》None，n=4或者5的时候，删除的就应该是第一个节点，所以返回head.next就好

            return head.next

        while fast.next:

            fast = fast.next

            slow = slow.next

        slow.next = slow.next.next

        return head