题目链接: https://leetcode.com/problems/reverse-linked-list-ii/description/
Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.
Given a linked list, remove the $n_{th}$ node from the end of list and return its head.
题目大意：对于一个给出的数字串，输出其可能的在手机键盘上代表的字符串，如对于数字串“23”。
其可能代表的字符串便有”ad”, “ae”, “af”, “bd”, “be”, “bf”, “cd”, “ce”, “cf”
给于一个整型数字范围在1-3999，将它转化为罗马数字