019 remove nth node from end of list
19. Remove Nth Node From End of List
题目：
https://leetcode.com/problems/remove-nth-node-from-end-of-list/
难度: Medium
AC击败了95.80%的Python用户，技巧 dummy head 和双指针。
切记最后要返回dummy.next而不是head，因为有这样一种情况，删掉节点后linked list空了，那返回head的话结果显然不同。如：
输入链表为[1], n = 1, 应该返回None而不是[1]
class Solution(object):
def removeNthFromEnd(self, head, n):
"""
:type head: ListNode
:type n: int
:rtype: ListNode
"""
dummy = ListNode(-1)
dummy.next = head
p, q = dummy, dummy
for i in range(n):
q = q.next
while q.next:
p = p.next
q = q.next
p.next = p.next.next
return dummy.next
