input: Write a python function to find the first non-repeating character in a string.
  assert first_non_repeating_char("swiss") == 'w'
  assert first_non_repeating_char("programming") == 'p'
  assert first_non_repeating_char("aabbcc") == -1
output: def first_non_repeating_char(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 -1