Remove Nth Node From End of List

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