I'm not sure if this is intentional or not but the "official" solution to today's challenge appears to have a bug.<p><a href="https://github.com/opentable/ot-daily-algorithm/commit/e811a1c2d500df90148164806411af76fa52c179#diff-9bd97815e9a3b60534ac901d300b3dcb" rel="nofollow">https://github.com/opentable/ot-daily-algorithm/commit/e811a...</a><p>It will return true even when the tree is not a valid BST. Consider the case where a leaf in the left subtree contains a value higher than the root of the tree.