Maximum Length of a Concatenated String with Unique Characters

You are given an array of strings arr. A string s is formed by the concatenation of a subsequence of arr that has unique characters.

Return the maximum possible length of s.

Constraints: 1 <= arr.length <= 16. 1 <= arr[i].length <= 26. arr[i] contains only lowercase English letters.

Example 1:

Input: arr = ["un","iq","ue"]
Output: 4
Explanation: Valid concatenations include "uniq" ("un" + "iq") and "ique" ("iq" + "ue"). Maximum length is 4.

Example 2:

Input: arr = ["cha","r","act","ers"]
Output: 6
Explanation: Possible longest valid concatenations are "chaers" ("cha" + "ers") and "acters" ("act" + "ers").

Example 3:

Input: arr = ["abcdefghijklmnopqrstuvwxyz"]
Output: 26