Find smallest substring containing all characters - Julia reviewed the code, gave out brute force solution, major hint to write brute force solution. Python code

Find smallest substring containing all characters

def get_shortest_unique_substring(arr, str):
  # if either arr or string is empty, return ''
  # iterate through the array, for each item
  # see if that item exists in the string
  # if so, store it in a dict, with the letter as the key and the count as the value
  substrings = []
  start_idx = 0
  
  while start_idx < len(str) - 1:
    end_idx = start_idx
    for end_idx < len(str) - 1:
      substring = str[start_idx: end_idx]
      substrings.append(substring)
    start_idx += 1
  
  output_str = ''
  for substring in substrings:
    found = true  // * 
    for letter in array:
      if not substring.contains(letter):
        found = false
        break
      else:
        //if len(substring) < output_string:  // one letter - make containing all letters -> **
        //  output_str = substring  // ***
    
    // find all letters
    if(found)  // ****
      substring containing -> // *****
  return output
  
* is the line of code added or commented out by Julia         
  
  Julia gave out major hint to write brute force solution after 10 minutes in the following:
  
  for(start_idx = 0; start_idx < len(str); start_idx++)
  {
     for(end_idx = start_idx; end_idx < len(str);end_idx++)  
    {
      substring = str[start_idx, end_idx]
    }
  }
  
  Julia gave out the brute force solution analysis first in the following after 10 minutes mock interview:
  
  [z,y,x] 
  
  xyyzyzyx 
  -> brute force 
  
  yyzy -> z, y, x no
  yyzyzyx -> z, y, x, yes -> 7, what is minimum? 
  
  Find all possible solution 
  
  Let us say that string length is n, what is possible substring in total?
  
  start position O(n)
  end position O(n)
  what is substring option? O(n * n)
  
  what is time complexity to find keys in substring? linear, O(n)
  
  O(n * n * n)