Keep One of Each Letter in the Smallest Subsequence
Company: Amazon
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: medium
Interview Round: Other
### Problem
Delete characters from s so every distinct input letter appears exactly once, while the remaining sequence is lexicographically smallest among all such subsequences. Retain relative order; arbitrary rearrangement is not allowed.
```cpp
string removeDuplicateLetters(string s);
```
### Examples
```text
s = "abacb" -> "abc"
s = "bbcaac" -> "bac"
```
### Constraints
- Length is 1 through 10000.
- Characters are lowercase English letters.
Overview: Keep One of Each Letter in the Smallest Subsequence. Delete characters from s so every distinct input letter appears exactly once, while the remaining sequence is lexicographically smallest among all such subseque
