input: Write a function to find the first non-repeating character in a given list. If no such character exists, return None. If the list is empty, return None.
  def first_non_repeating_char(lst):
      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
  
  assert first_non_repeating_char([1, 2, 3, 4, 4, 3, 2, 1]) == None
  assert first_non_repeating_char([5, 6, 7, 8, 8, 7, 6, 5, 9]) == 9
  assert first_non_repeating_char([10, 11, 12, 12, 13, 14, 13]) == 11
  assert first_non_repeating_char([]) == None
  assert first_non_repeating_char([1, 1, 1, 2, 2, 3, 4, 4, 5]) == 3
output: def first_non_repeating_char(lst):
      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