func_code: def swap_unique_keys_values(x):
      x1 = {}
      x2 = {}
      sorx = sorted(list(d.items()), reverse=True)
      for keys, values in sorx:
          if values not in x1:
              x1[values] = keys
          else:
              x1[values] = 'N'
      sorx2 = sorted(list(x1.items()), reverse=True)
      for keys, values in sorx2:
          if values != 'N':
              x2[keys] = values
      return x2