add swap() function.
add swap() function.

and fixes comment of swap member function.

hiroyuki-seki/hryky-codebase,hiroyuki-seki/hryky-codebase,hiroyuki-seki/hryky-codebase,hiroyuki-seki/hryky-codebase,hiroyuki-seki/hryky-codebase,hiroyuki-seki/hryky-codebase