https://stackoverflow.com/questions/13524011/i-do-not-understand-the-concept-of-non-deterministic-turing-machine

https://en.wikipedia.org/wiki/Subset_sum_problem

https://en.wikipedia.org/wiki/Computational_complexity_theory

https://www.quora.com/What-are-P-NP-NP-complete-and-NP-hard

https://en.wikipedia.org/wiki/Non-deterministic_Turing_machine

http://www.cs.odu.edu/~toida/nerzic/390teched/tm/othertms.html

http://www.cs.rpi.edu/~goldberg/14-CC/02-ndt.pdf

http://jeffe.cs.illinois.edu/teaching/algorithms/notes/models/09-nondeterminism.pdf

non-deterministic-turing-machine