project_euler
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https://projecteuler.net/problem=1 C++
https://projecteuler.net/problem=2 C++ capture by value in lambda are const and thus the captured value cannot be modified, that is, if you want a stateful function, you must capture by reference. what if the deferenced object being modified both inside and outside of the lambda? python use generator to generate a potentially infinite sequence by recursive relation. -- a stateful function with two hyper-parameters.