Smallest Subsequence With Each Letter Once

You are given a string s of lowercase English letters.

A subsequence of s keeps some of its characters, in their original order, and drops the rest.

Return the lexicographically smallest subsequence of s that contains every distinct letter of s exactly once.

Examples

Example 1

Input: s = "bcabc"
Output: "abc"
Explanation: The distinct letters of s are a, b, c. Among all subsequences that use each of them exactly once, "abc" is the smallest in lexicographical order.

Example 2

Input: s = "cbacdcbc"
Output: "acdb"
Explanation: The distinct letters of s are a, b, c, d. Among all subsequences that use each of them exactly once, "acdb" is the smallest in lexicographical order.