input: Write a python function to strip whitespace from a string.
  assert strip_whitespace('Hello World') == 'HelloWorld'
  assert strip_whitespace('Python Programming') == 'PythonProgramming'
  assert strip_whitespace('  Leading and trailing  ') == 'Leadingandtrailing'
  assert strip_whitespace('NoSpaces') == 'NoSpaces'
output: def strip_whitespace(s):
      return s.replace(" ", "")