This exercise has no Swift stubs and there is no way of knowing how it's suppose to create the invalid binary search trees with the provided input values.

Java 8 bugged, no class to work with

Single line solution

def checkBST(root, min_v =-1 , max_v = 10001):
    return True if root is None else (root.data < max_v and root.data > min_v  and checkBST(root.left, min_v, root.data) and checkBST(root.right,root.data, max_v))

how does the node structure look like?

Kotlin is broken, too. No hidden code at all, no checkBST function to start with.