链表操作，只能遍历一遍然后。

用双指针

# Definition for singly-linked list.# class ListNode(object):#     def __init__(self, x):#         self.val = x#         self.next = Noneclass Solution(object):    def removeNthFromEnd(self, head, n):        """        :type head: ListNode        :type n: int        :rtype: ListNode        """        left=right=head        for i in range(n):            right=right.next        if right is None:   return head.next    #special        while right.next is not None:            right=right.next            left=left.next                    left.next=left.next.next        return head