Given an array of unique characters arr and a string str, implement a function getShortestUniqueSubstring that finds the smallest substring of str containing all the characters in arr. Return "" (empty string) if such a substring doesn’t exist.

This algorithm is used.

import collections import sys  def get_shortest_unique_substring(substring, string):   def trim_left(substring, window, count):     ''' find how much left pointer can be moved in a window where all substring is present'''     for left, c in enumerate(window):       if c in count and count[c] > 1:         count[c] -= 1       elif c not in count:         continue       else:         return left     return 0    substring = set(substring)   ''' validating input '''   if not substring or not string:     return ""   if len(substring) == len(string) and substring == set(string):     return string    window, min_window, min_window_len = [], [], sys.maxint   left, right, count = 0, 0, collections.Counter()   # to check if the initial minimum window has been found which has all the substring   substr_found = set(substring)    for right, c in enumerate(string):     if c in substring:       count[c] += 1       if c in substr_found:         substr_found.remove(c)       # window has been found and now trim left pointer as much as possible maintaining the invariant that substr is still present between left and right pointers       if not substr_found:         left += trim_left(substring, string[left:right+1], count)         # update the minimum window         if right - left + 1 < min_window_len:           min_window_len, min_window = right - left + 1, string[left:right+1]         # we can't find better window than the length of substr         if len(min_window) == len(substring):           return "".join(min_window)   return "".join(min_window)