input: Write a function to find the first non-repeating character in a given list. If all characters are repeating then return None.
  assert first_non_repeating_char([1, 2, 3, 4, 4, 3, 2, 1])==None
  assert first_non_repeating_char([5, 2, 3, 5, 4, 3, 2, 1])==4
output: def first_non_repeating_char(lst):
      count = {}
      for char in lst:
          if char in count:
              count[char] += 1
          else:
              count[char] = 1
      for char in lst:
          if count[char] == 1:
              return char
      return None