All Unique Permutations of a String

Question

## Problem

Given a string `s` that may contain **duplicate characters**, return all unique permutations.

## Example
```
s = "ABA"
Output: ["AAB", "ABA", "BAA"]
```

## Constraints
- 1 ≤ s.length ≤ 8
- Characters are uppercase English letters

## Approach
Backtracking with a frequency map to avoid duplicate branches. Sort characters first, then skip if same character was already used at the same recursion depth.

## Follow-up
Return permutations in lexicographic order without sorting the final output list.