# Definition for singly-linked list.class ListNode:    def __init__(self, x):        self.val = x        self.next = Noneclass Solution:    # @return a ListNode    def removeNthFromEnd(self, head, n):        if head==None:            return         pre_head=ListNode(0)        pre_head.next=head        ruler_end=pre_head        ruler_head=pre_head        while n:            ruler_end=ruler_end.next            n-=1        while ruler_end.next!=None :            ruler_end=ruler_end.next            ruler_head=ruler_head.next        temp=ruler_head.next        ruler_head.next=temp.next        return pre_head.next