Status: IN
Per-problem code duplication (TreeNode definitions, tree builders, test helpers) carries zero maintenance cost as long as every copy is correct — there is no shared module to fix once, but there is also nothing to break.
duplication is free precisely until a copy contains a bug — the O(n) list.pop(0) in the tree builder is duplicated across every tree problem, so fixing it requires N edits instead of one, turning zero-coupling into a liability
Depends on (SL): duplication-over-shared-infrastructure, self-contained-solution-with-local-treenode
Unless: build-helper-uses-list-pop-zero