serialize_tree

#include <vector>
#include <memory>
#include <iostream>

struct node
{
    std::vector<std::shared_ptr<node>> children;
};

int serialize_tree(std::vector<std::vector<int>> &result, std::shared_ptr<node> root)
{
    int index = result.size();
    result.push_back({});

    for (std::shared_ptr<node> child : root->children)
    {
        result.at(index).push_back(serialize_tree(result, child));
    }

    return index;
}

std::shared_ptr<node> generate_tree(int depth)
{
    std::shared_ptr<node> root = std::make_shared<node>();

    for (int i = 0; i < depth; i++)
    {
        root->children.push_back(generate_tree(depth - 1));
    }

    return root;
}

void print_serialized_tree(std::vector<std::vector<int>> tree)
{
    for (size_t i = 0; i < tree.size(); i++)
    {
        for (size_t j = 0; j < tree[i].size(); j++)
        {
            if (j != 0)
                std::cout << ' ';
            std::cout << tree[i][j];
        }

        std::cout << std::endl;
    }
}

int main()
{
    std::shared_ptr<node> root = generate_tree(5);

    std::vector<std::vector<int>> result;
    serialize_tree(result, root);

    print_serialized_tree(result);
    return 0;
}