input: def first_non_repeating_char(lst):
      """Find the first non-repeating character in a given list.
      >>> first_non_repeating_char([1, 2, 3, 4, 4, 3, 2, 1])
      None
      >>> first_non_repeating_char(['a', 'b', 'c', 'a', 'b'])
      'c'
      """
output:     char_count = {}
      for char in lst:
          if char in char_count:
              char_count[char] += 1
          else:
              char_count[char] = 1
      
      for char in lst:
          if char_count[char] == 1:
              return char
      
      return None