It would be nice if someone can push missing descriptions for those problems listed. I really appreciate that. Additionally, please do not use the source code for any other purpose than education, especially plagiarism.

dooms_day

• Fraction class and Linear Algebra class
• Markov chain, LU Decomposition, Inverse Matrix

find_the_access_codes

• pure Array problems

bomb_baby

• BigInteger and recursive similar to GCD

free_the_bunny_prisoners

• Combinations theory, Lexicographical order

escape_pods

• Maximum flow with multiple sources and sinks
• https://en.wikipedia.org/wiki/Maximum_flow_problem