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 <= 1000
• s consists of lowercase English letters.

 

Note: This question is the same as 316: https://leetcode.com/problems/remove-duplicate-letters/