When a level has an odd number of nodes the last one has no partner - our rule is to promote it up to the next level unchanged. Today you handle that odd node.
Extend pairUp so a level with an odd count promotes its lone last node up unchanged.
Real datasets are rarely a power of two, so most levels eventually hit an odd count with one hash left over. You need a rule, and there are two common ones. This project promotes the lone node: it moves up to the next level unchanged, waiting to be paired higher up. (Bitcoin instead duplicates the last node and hashes it with itself - simpler in one way, but it caused a real-world malleability bug. Pick one rule and be consistent; we promote.)
The important discipline is that this is the only change: the even branch is untouched, so the four-leaf result from last lesson is exactly the same. A promoted node carries no extra prefix or wrapper - it is literally the same hash one level up. That simplicity is what keeps proofs and diffs over odd trees tractable later.
func pairUp(level []Hash) []Hash {var next []Hashi := 0for ; i+1 < len(level); i += 2 {next = append(next, HashNode(level[i], level[i+1]))}if i < len(level) { // one node left with no partnernext = append(next, level[i]) // promote it unchanged}return next}