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.
  
  def capitalize_string(str):
      if not str:
          return ""
      words = str.split(' ')
      capitalized_words = [word.capitalize() for word in words]
      return " ".join(capitalized_words)
  
  assert capitalize_string("this is an example")=='This Is An Example'
  assert capitalize_string("")==''
  assert capitalize_string("hello world")=='Hello World'
  assert capitalize_string("a b c d e f")=='A B C D E F'
  assert capitalize_string("single")=='Single'
output: def capitalize_string(str):
      if not str:
          return ""
      words = str.split(' ')
      capitalized_words = [word.capitalize() for word in words]
      return " ".join(capitalized_words)