Using pragma once instead of include guards

[svaderia]

Instead of having #ifndef to guard against the multiple includes, #pragma once can be used. Looks a bit cleaner that way.

[yosupo06]

Thanks for suggesting!

Yes, #pragma once is more clear way to write include guard, but we use #ifndef Intentionally. #pragma once isn't C++ standard (https://en.wikipedia.org/wiki/Pragma_once ). We don't use non-standard function if there is no special reason.