1#include <iostream>
2#include <vector>
3#include <string>
4#include <unordered_set>
5
6using namespace std;
7
8bool isUnique(string &s) {
9    vector<int> chars(26, 0);
10    for (char c : s) {
11        if (chars[c - 'a']++ > 0) return false;
12    }
13    return true;
14}
15
16void backtrack(vector<string> &arr, string current, int index, int &max_length) {
17    if (!isUnique(current)) return;
18    if (current.size() > max_length) {
19        max_length = current.size();
20    }
21    for (int i = index; i < arr.size(); i++) {
22        backtrack(arr, current + arr[i], i + 1, max_length);
23    }
24}
25
26int maxLength(vector<string> &arr) {
27    int max_length = 0;
28    backtrack(arr, "", 0, max_length);
29    return max_length;
30}
31
32int main() {
33    vector<string> arr = {"un", "iq", "ue"};
34    cout << maxLength(arr) << endl;
35    return 0;
36}

using System.Collections.Generic;

public class Solution {
    public int MaxLength(IList<string> arr) {
        return BitmaskHelper(arr, 0, 0);
    }

    private int BitmaskHelper(IList<string> arr, int index, int bitmask) {
        int maxLength = 0;
        for (int i = index; i < arr.Count; i++) {
            int newBitmask = 0;
            bool valid = true;
            foreach (char c in arr[i]) {
                int mask = 1 << (c - 'a');
                if ((bitmask & mask) != 0) {
                    valid = false;
                    break;
                }
                newBitmask |= mask;
            }
            if (valid) {
                maxLength = Math.Max(maxLength, BitmaskHelper(arr, i + 1, bitmask | newBitmask) + arr[i].Length);
            }
        }
        return maxLength;
    }

    public static void Main() {
        var arr = new List<string> { "un", "iq", "ue" };
        var solution = new Solution();
        Console.WriteLine(solution.MaxLength(arr));
    }
}