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.
Note:
Given n will always be valid.
Try to do this in one pass.
解决
两个指针，一个指针先走 n 步，然后另一个指针也开始走，一直到前一个指针的下一结点为null，此时后一指针的下一结点就是要删除的
结点
