input: Write a function to capitalize the first character of each word in a given string. The function should take a string as input and return the capitalized string.
  assert capitalize_string("this is an example")=="This Is An Example"
  assert capitalize_string("")==""
  assert capitalize_string("hello world")=="Hello World"
  assert capitalize_string("THIS IS ALL CAPS")=="This Is All Caps"
  assert capitalize_string("123 home street")=="123 Home Street"
output: def capitalize_string(s):
      return ' '.join(word.capitalize() for word in s.split())