int *inorder_rec( struct node *root, int *start, int *end )
{
    if ( !root )
        return start;

    start = inorder_rec( root->left, start, end );

    if ( !start || start + 1 > end )
        return NULL;

    *start++ = root->value;

    assert( start <= end );

    return inorder_rec( root->right, start, end );
}

struct values inorder( struct heap *heap, struct node *root )
{
    struct values res = { .data = ( int * ) heap->start };
    int *end = inorder_rec( root, res.data, ( int * ) heap->end );
    if ( end )
    {
        res.length = end - res.data;
        heap->start = ( char * ) end;
    }
    else
        res.data = NULL;

    return res;
}