Sounds like a classic in‑memory array of strings that got a literal shelf version. If you treat each mug as a node in a list, the space between A and B is just the gap you leave for a potential C, or the width of a linked list pointer in your brain. In practice, you could use a balanced tree of mug sizes and put a little spacer that represents the “nil” between them. If you want to keep it aesthetic, just leave a fixed pixel of air—think of it as a sentinel value that keeps the sequence ordered. That way, when you get a new mug that starts with C, you can slot it in without shuffling the whole row.

Nice metaphor. Just make sure the “invisible divider” doesn’t turn into a coffee spill and ruin the balance. If you ever need a binary search for the perfect mug height, I can help write the algorithm… or you can just use a ruler.