Последний элемент нулл

int main()
{
    std::vector<int> v = { 1, 5, 3, 8, 22, 69, 7, 43, 26 };
    branch* tree = vector_to_tree(v);
 
    std::copy(v.begin(), v.end(), std::ostream_iterator<int>(std::cout, ", "));
    std::cout << "\n";
 
    while (tree)
    {
        std::cout << " - " << tree->value << "\n";
        tree = next(tree);
    }
}

struct branch
{
    int value;
    branch* parent;
    branch* left;
    branch* right;
};
 
branch* add(int value, branch* where = nullptr)
{
    branch* b = new branch;
    b->value = value;
    b->left = nullptr;
    b->right = nullptr;
    b->parent = nullptr;
 
    if (!where) return b;
 
    b->parent = where;
    if (where->value < value)
        if (where->right)
            add(value, where->right);
        else
            where->right = b;
 
    if (where->value >= value)
        if (where->left)
            add(value, where->left);
        else
            where->left = b;
 
    return b;
}
 
 
branch* min(branch* tree)
{
    while (tree->left)
        tree = tree->left;
 
    return tree;
}
 
branch* max(branch* tree)
{
    if (!tree) return nullptr;
 
    while (tree->right)
        tree = tree->right;
 
    return tree;
}
 
void remove(branch* to_delete)
{
    if (!to_delete->left && !to_delete->right)
        to_delete = nullptr;
    else
        to_delete = min(to_delete->left);
}
 
branch* next(branch* b)
{
    if (b->right)
        return min(b->right);
    
    if (b->left)
        return max(b->right);
 
    if (b->parent)
        return b->parent;
 
    return nullptr;
}
 
branch* prev(branch* b)
{
    if (b->left)
        return max(b->left);
 
    return nullptr;
}
 
branch* vector_to_tree(const std::vector<int>& v)
{
    branch* head = new branch;
    head->value = v.at(0);
    head->left = nullptr;
    head->right = nullptr;
    head->parent = nullptr;
 
    branch* tree = head;
    for (int i = 1; i < v.size(); ++i)
        add(v[i], tree);
 
    return head;
}