public boolean isBST(BinaryTreeNode<Integer> root) {
if (root == null) return true;
boolean leftValid = root.getLeft() == null ? true : max(root.getLeft()) <= root.getValue();
boolean rightValid = root.getRight() == null ? true : max(root.getRight()) > root.getValue();
if (leftValid && rightValid) {
return isBST(root.getLeft()) && isBST(root.getRight());
}
return false;
}
public Integer max(BinaryTreeNode<Integer> node) {
while (node.getRight() != null) {
node = node.getRight();
}
return node.getValue();
}
