Maximize concatenation length with unique chars

## Coding: Maximum-length unique-character string You are given an array of strings `arr`. You may choose **any subset** of these strings and concatenate them in any order. The resulting concatenated string must contain **only unique characters** (i.e., no character appears more than once in the final string). Return the **maximum possible length** of such a concatenated string. ### Notes / constraints - Strings contain lowercase English letters `a`–`z`. - You cannot reorder characters *within* a string, but you may choose the order of concatenation by choosing strings in some order. - If a string contains duplicate characters internally (e.g., `"aa"`), it can never be part of a valid concatenation. ### Example - Input: `arr = ["un","iq","ue"]` - Output: `4` (choose `"un" + "iq"` or `"iq" + "ue"`)