Status: IN
The TreeNode infrastructure (shared via inline copies across 400+ test files) provides both correct and efficient tree construction from level-order arrays.
Tree construction is correct but O(n^2) due to list.pop(0); replacing with deque.popleft() would validate this claim
Depends on (SL): treenode-shared-definition-in-preorder, build-tree-level-order-convention, build-tree-level-order
Unless: build-helper-uses-list-pop-zero