LeetCode:Remove Nth Node From End of List

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.

这个题很早就做过了，不难，不过小细节很多，最后还是调试通过的，汗！
注意点：用一个变量记录node个数，当n==number of nodes,时删除head