一招吃遍力扣四道题，妈妈再也不用担心我被套路啦～
- 去除重复字母(困难)
- 拼接最大数(困难)
- 移掉 K 位数字(中等)
- 不同字符的最小子序列（中等）
https://leetcode-cn.com/problems/remove-duplicate-letters/solution/yi-zhao-chi-bian-li-kou-si-dao-ti-ma-ma-zai-ye-b-4/
https://leetcode-cn.com/problems/remove-duplicate-letters/solution/yi-zhao-chi-bian-li-kou-si-dao-ti-ma-ma-zai-ye-b-4/