input: Write a function to find the first non-repeating character in a given list. If no such character exists, return None.
  
  assert first_non_repeating_char([1, 2, 3, 4, 4, 3, 2, 1]) == None
  assert first_non_repeating_char([4, 1, 2, 2, 1, 4, 3]) == 3
  assert first_non_repeating_char([7, 8, 8, 7, 9]) == 9
  assert first_non_repeating_char([5, 5, 6, 6, 7, 8]) == 7
  assert first_non_repeating_char([]) == None
output: def first_non_repeating_char(lst):
      count = {}
      for item in lst:
          if item in count:
              count[item] += 1
          else:
              count[item] = 1
      for item in lst:
          if count[item] == 1:
              return item
      return None