Given a string s, return the lexicographically smallest subsequence of s that contains all the distinct characters of s exactly once.
Example 1:
Input: s = "bcabc" Output: "abc"
Example 2:
Input: s = "cbacdcbc" Output: "acdb"
Constraints:
1 <= s.length <= 1000sconsists of lowercase English letters.
Note: This question is the same as 316: https://leetcode.com/problems/remove-duplicate-letters/