input: Write a function to find the first non-repeating character in a given list. If there is no non-repeating character, 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, 6, 7, 8])==5
  assert first_non_repeating_char([10, 11, 12, 12, 13, 13, 11])==10
  assert first_non_repeating_char([20, 21, 22, 20, 21, 23])==22
  assert first_non_repeating_char([])==None
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