input: Write a python function to find the first non-repeating character in a string.
  assert first_non_repeating_character("swiss") == "w"
  assert first_non_repeating_character("releveler") == "v"
  assert first_non_repeating_character("aabbcc") == None
output: def first_non_repeating_character(s):
      char_count = {}
      
      for char in s:
          if char in char_count:
              char_count[char] += 1
          else:
              char_count[char] = 1
              
      for char in s:
          if char_count[char] == 1:
              return char
      
      return None