the method receives a root node of a binary tree and should return the sum of all node's items
that are bigger than the number n (I did it with double, but it can be ints just the same)

static double sumOfBiggerThan(TreeNode node, double n) {
            if (node == null) return 0;

            double temp = 0;

            if (node.item > n) temp = n;

            temp += sumOfBiggerThan(node.left, n);
            temp += sumOfBiggerThan(node.right, n);

            return temp;
}