input: Write a python function to find the first non-repeating character in a string.
  assert non_repeating_char("the quick brown fox jumps over the lazy dog") == 't'
  assert non_repeating_char("aabbccddeeffgghhiijjkkllmmnnooppqqrrssttuuvvwwxxyyzz") == None
  assert non_repeating_char("hello") == 'h'
output: def non_repeating_char(s):
      from collections import Counter
      count = Counter(s)
      for char in s:
          if count[char] == 1:
              return char
      return None