Ad Hoc Problems
`Ad hoc' problems are those whose algorithms do not fall into standard categories with well-studied solutions. Each ad hoc problem is different; no specific or general techniques exist to solve them.
Of course, this makes the problems the `fun' ones, since each one presents a new challenge. The solutions might require a novel data structure or an unusual set of loops or conditionals. Sometimes they require special combinations that are rare or at least rarely encountered.
Ad hoc problems usually require careful reading and usually yield to an attack that revolves around carefully sequencing the instructions given in the problem.
Ad hoc problems can still require reasonable optimizations and at least a degree of analysis that enables one to avoid loops nested five deep, for example.
More ad hoc problems appear on this web site than any other kind of problem. Always be ready for an ad hoc problem if you can not classify a problem as one of the other standard types (to be listed later).