The leitner system has been invented in 1970 https://en.wikipedia.org/wiki/Leitner_system
but Adaptively-spaced repetition which is the base of Leitner system has been registered in google patents in 2012 here http://www.google.com/patents/US20120322043
the first question is "everything in google patents page is really patented ?" and the second question is "I've developed an application which works based on Leitner systems ,now I'm wondering if I'm breaking the law or not?"