8
  /
 3    2
/
1  2

public boolean isValid()
{
    return isValid(root);
}

public boolean isValid(BinaryNode<E> node)
{
    if (node == null)
        return true;

    int sum = sumOfSub(node.getLeft()) + sumOfSub(node.getRight());

    if (node.getLeft() != null && node.getRight() != null)
    {
        if ((Integer) node.getData() != sum)
        {
            return false;
        }
    }
    else
        return true;

    return isValid(node.getLeft()) && isValid(node.getRight());
}

private int sumOfSub(BinaryNode node)
{
    if (node == null) return 0;
    int value = (Integer) node.getData();
    return value + sumOfSub(node.getLeft()) + sumOfSub(node.getRight());
}