居然一次AC了，虽然是道极水的题。考虑好一些特殊条件，比如n小于等于0， root等于NULL即可。

ListNode *removeNthFromEnd(ListNode *head, int n) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        if(n <= 0 || head == NULL)            return head;        int len = 0;        ListNode *tmp = head;        while(tmp != NULL){            len++;            tmp = tmp->next;        }        if(n > len)            return head;        if(n == len)            return head->next;        len = len - n;        tmp = head;        while(len > 1){            len--;            tmp = tmp->next;        }        tmp->next = tmp->next->next;        return head;    }