MedianFillFilter.cpp -> function cleanedMask() 's morphological operational question

[silencht]

in grid_map_filters/src/MedianFillFilter.cpp,

  cv::dilate(inputMask, cleanedInputMask, element);
  cv::erode(cleanedInputMask, cleanedInputMask, element);
  cv::erode(inputMask, cleanedInputMask, element);
  cv::dilate(cleanedInputMask, cleanedInputMask, element);

My idea was to write it like this，

  cv::dilate(inputMask, cleanedInputMask, element);
  cv::erode(cleanedInputMask, cleanedInputMask, element);
  cv::erode(cleanedInputMask, cleanedInputMask, element);
  cv::dilate(cleanedInputMask, cleanedInputMask, element);

Is there a problem with the third line of code here? Or is there a problem with my understanding. Can you explain what this part of the code does? Thanks!