input: Write a python function to find the first non-repeating character in a string.
  assert non_repeating_character('Hello, world!')=='H'
  assert non_repeating_character('aabbccddeeffg')=='g'
  assert non_repeating_character('aabbcc')=='No result'
output: def non_repeating_character(s):
      char_count = {}
      for char in s:
          char_count[char] = char_count.get(char, 0) + 1
          
      for char in s:
          if char_count[char] == 1:
              return char
      return 'No result'