Remove Nth Node From End of List

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;
    }