1using System;
2using System.Collections.Generic;
3
4public class Solution {
5    public int MaxLength(IList<string> arr) {
6        return Backtrack(arr, "", 0);
7    }
8
9    private int Backtrack(IList<string> arr, string current, int index) {
10        if (!IsUnique(current)) return 0;
11        int maxLength = current.Length;
12        for (int i = index; i < arr.Count; i++) {
13            maxLength = Math.Max(maxLength, Backtrack(arr, current + arr[i], i + 1));
14        }
15        return maxLength;
16    }
17
18    private bool IsUnique(string s) {
19        int[] chars = new int[26];
20        foreach (char c in s) {
21            if (chars[c - 'a']++ > 0) return false;
22        }
23        return true;
24    }
25
26    public static void Main() {
27        var arr = new List<string> { "un", "iq", "ue" };
28        var solution = new Solution();
29        Console.WriteLine(solution.MaxLength(arr));
30    }
31}