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