input: from typing import List
  
  def first_non_repeating_char(lst: List[int]) -> int:
      """
      Find and return the first non-repeating character in a given list.
      If all characters are repeated, return None.
      >>> first_non_repeating_char([1, 2, 3, 4, 4, 3, 2, 1])
      None
      >>> first_non_repeating_char([1, 2, 3, 4, 4, 3, 2, 1, 5])
      5
      """
output:     count_dict = {}
      for num in lst:
          if num in count_dict:
              count_dict[num] += 1
          else:
              count_dict[num] = 1
  
      for num in lst:
          if count_dict[num] == 1:
              return num
  
      return None