input: Write a function to capitalize the first character of each word in a given string and return the modified string. If the given string is empty then return the empty string.
  assert capitalize_string("this is an example")=="This Is An Example"
  assert capitalize_string("another example")=="Another Example"
  assert capitalize_string("singleword")=="Singleword"
  assert capitalize_string("")==""
output: def capitalize_string(s):
      if not s:
          return s
      return ' '.join(word.capitalize() for word in s.split())