struct tree_node* successor(struct tree_node *root) {
    if (root == NULL)
        return;


    if (root->right_child != NULL)
    {
        struct tree_node *iterator = root->right_child;
        while (iterator->left_child != NULL)
        {
            iterator = iterator->left_child;
            return iterator;
        }
    }
    else
    {
        struct tree_node *iterator = root;
        while (root->parent->right_child == root)
        {
            iterator = root->parent;
        }
        if (root->parent != NULL)
        {
            return root->parent;
        }
        else
        {
            printf("No successor!");
            return NULL;
        }
    }
}