Fix ruby version.
Format code.
Refactor: use map.
Rename methods.
Refactor: use product.
Refactor: avoid shadowing.
Refactor: refine swap_male.
Refactor: refine flat_arr.
Refactor: according to method name.
Refactor: rename argument name.
Refactor spec.
Merge specs.
Organize directories.
Organize filenames.
Organize directories.