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