XXOO - free

     

XXOO comes from Yin Liu. It's kind of like Lights Out where the objective is to get all the tiles to 0. You do this by drawing rectangles of any size within the grid. Whatever's inside that rectangle will switch.
There are 200 levels. Ads started popping up around level 40 - at least for me.  I solved 104 levels in about an hour and a half.  Once you get going, these go pretty fast.  The ones I did were mostly 3-step puzzles.  Around level 90, they became 4-step.
I wish I could finish these, but I just don't have the time.
https://itunes.apple.com/gd/app/xxoo/id955324519?mt=8