AlphaShape doesn't work for this?

I tried all possible alphas, for which the alphaShape changes, but I couldn't capture the interior of this polygon. Maybe it's because of the sharp 90-degree-edges?

The Input data was too many unordered points defining the boundary of the polygon.

...

https://www2.pic-upload.de/img/33311040/sketch-1497040111343.png

[Walter Roberson]

I think you missed posting an image? And perhaps the data too?