19. Remove Nth Node From End of List（medium）

Given a linked list, remove the nth node from the end of list and return its head.

For 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.

思路：遍历一次若链表长度小于n return 否者
找到倒数第n个节点并记录他的父节点