input: Write a python function to search for a keyword in a dictionary's values and return the corresponding key.
  assert search_string({'a': 'apple', 'b': 'banana', 'c': 'cherry'}, 'banana') == 'b'
  assert search_string({'x': 'x-ray', 'y': 'yellow', 'z': 'zebra'}, 'zebra') == 'z'
  assert search_string({'key1': 'value1', 'key2': 'value2'}, 'value3') == 'Not found'
output: def search_string(d, keyword):
      for key, value in d.items():
          if value == keyword:
              return key
      return 'Not found'