input: 
  from typing import List
  
  
  def first_non_repeating_char(lst: List[int]) -> int:
      """Find the first non-repeating character in a given list.
      >>> first_non_repeating_char([1, 2, 3, 4, 4, 3, 2, 1])
      3
      >>> first_non_repeating_char([5, 5, 5])
      None
      >>> first_non_repeating_char([6, 7, 8])
      6
      """
      dct = {}
      for l in lst:
          if l in dct:
              dct[l] += 1
          else:
              dct[l] = 1
      for l in lst:
          if dct[l] == 1:
              return l
      return None
  
output:     dct = {}
      for l in lst:
          if l in dct:
              dct[l] += 1
          else:
              dct[l] = 1
      for l in lst:
          if dct[l] == 1:
              return l
      return None