Click on a number from 1 to 5 for starter levels or click on '+' button for 8x8 and higher levels(6 thru 15). Press 'n' key to advance to next level, 'p' key to go back to previous level. ('e' key takes you to final level) Create a "save point" by pressing 's' key while solving a puzzle. Press 'r' key to restore puzzle to last "save point".(only one "save point" is stored in memory), Click on a box to circle, click again to darken, once more to un-darken. If you're new to Hitori, you can get instruction and solving tips here: https://www.conceptispuzzles.com/index.aspx?uri=puzzle/hitori/techniques
A big THANK YOU to giorgia77 for a brilliant original project! 12/23/20 - added 'e' key to go directly to final level, made level 15 a 9x9