Status: IN
Tree solutions combine closure-based DFS (capturing mutable result variables from the enclosing scope for side-effect accumulation) with postorder return values (propagating subtree summaries upward for recursive composition), enabling single-pass O(n) tree computations that both accumulate a global answer and compose local results.
Closures handle cross-subtree accumulation while postorder handles intra-subtree composition — together they cover all tree computation needs
Depends on (SL): closure-dfs-pattern, postorder-accumulator-pattern