A long-time favourite way of diagraming the logic and flow of both problems before coding, and the code as an after-the-fact process is little known (and old *) technique called Warnier-Orr diagrams.

Personally, I find them infinitely preferable to UML. They are less complex and much easier to draw, read and understand IMO, but YMMV. There not perfect by any means, but little is.

* I first saw them described in a series of Byte magazine articles when I was a student. I just did a google and discovered these were in December 1977 and January 1978! {sigh} How time flies when your having fun.