BSTInvariant.elm

isValidBst : Tree comparable -> Bool
isValidBst tree =
    case tree of
        Empty ->
            True

        Node value left right ->
            all (\x -> x < value) left
                && all (\x -> x > value) right
                && isValidBst left
                && isValidBst right


all : (comparable -> comparable -> Bool) -> Tree comparable -> Bool
all condition tree =
    case tree of
        Empty ->
            True

        Node value left right ->
            condition value
                && all condition left
                && all condition right