using System;
using System.Collections.Generic;

class Solution
{
    public static string GetShortestUniqueSubstring(char[] keys, string source)
    {
      if(keys == null || source == null || source.Length == 0 || keys.Length ==0)
      {
        return "";
      }

      var dictionary = addToDictionary(keys);
      var hashSet = new HashSet<char>(keys);

      int left = 0;
      int minLength = int.MaxValue;
      int countOfKeys = 0;

      int length = source.Length;
      int startIndex = 0;

      for(int index = 0; index < length; index++) // axyyz
      {
        var current = source[index]; // 'a'

        var isInDict = dictionary.ContainsKey(current);
        if(!isInDict)
        {
          continue;
        }

        if(dictionary[current] == 1)
        {
          countOfKeys++;   // 1, 2, 3
        }

        dictionary[current]--; // x :0, y: -1, z:0

        // find substring containing all keys
        // "xyyz"
        // "xxyyz" -> "xyyz"
        // "xaxyz" -> "xyz"
        while(countOfKeys == keys.Length) // true  axyyz
        {
          var currentLength = index - left + 1; // 4 - 0 + 1 = 5, 4
          if(currentLength < minLength)
          {
            minLength = currentLength; // 5, 4, left = 1
            startIndex = left;
          }

          var leftChar = source[left]; // 'a', 'x'
          if(!hashSet.Contains(leftChar)) // true
             {
               left++; // 1
               continue;
             }

             dictionary[leftChar]++; // 1
             left++;  // 2
             if(dictionary[leftChar] == 1)
             {
               countOfKeys--;  // 2
             }
        }
      }

      return minLength == int.MaxValue? "" : source.Substring(startIndex, minLength);
    }

    private static Dictionary<char, int> addToDictionary(char[] keys)
    {
      var dict = new Dictionary<char, int> ();

      foreach(var item in keys)
      {
        dict.Add(item, 1);
      }

      return dict;
    }

    static void Main(string[] args)
    {
    }
}