两数之和#1(简单)
无重复字符的最长子串#3(中等)
https://leetcode.cn/problems/longest-substring-without-repeating-characters/
整数转罗马数字#12(中等)
罗马数字转整数#13(简单)
电话号码的字母组合#17(中等)
https://leetcode.cn/problems/letter-combinations-of-a-phone-number/
串联所有单词的子串#30(困难)
https://leetcode.cn/problems/substring-with-concatenation-of-all-words/