private void validateStructure(Node node) {
checkState(node.level >= 0);
if (node.left != null) {
validateBranchStructure(node, node.left, node.right, true);
validateStructure(node.left);
}
if (node.right != null) {
validateBranchStructure(node, node.right, node.left, false);
validateStructure(node.right);
}
}
