input: Write a function to capitalize the first character of each word in a given string. If the input string is empty, return an empty string. If the string contains only one word, capitalize its first character.
  assert capitalize_string("") == ""
  assert capitalize_string("word") == "Word"
  assert capitalize_string("this is an example") == "This Is An Example"
  assert capitalize_string("python coding") == "Python Coding"
  assert capitalize_string("a b c d e") == "A B C D E"
output: def capitalize_string(s):
      if not s:
          return ""
      return ' '.join(word.capitalize() for word in s.split())