19.py

'''
 Given a linked list, remove the n-th node from the end of list and return its head.

 Example:

 Given linked list: 1->2->3->4->5, and n = 2.

 After removing the second node from the end, the linked list becomes 1->2->3->5.

'''

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

classSolution(object):
defremoveNthFromEnd(self, head, n):
"""
 :type head: ListNode
 :type n: int
 :rtype: ListNode
 """
ifnothead:
returnNone

ref=head
whilen>0:
ref=ref.next
n-=1

ifrefisNone:
returnhead.next
else:
main=head
whileref.next:
main=main.next
ref=ref.next

main.next=main.next.next
returnhead