#include <cassert>
#include <filesystem>
#include <fstream>
#include <iostream>
#include <sstream>
#include <string>
#include <string_view>
#include <vector>
#include <algorithm>

using namespace std;
using filesystem::path;

path operator""_p(const char* data, std::size_t sz) {
    return path(data, data + sz);
}

void PrintTree(ostream& dst, const path& p, int offset) {
    vector<filesystem::directory_entry> directories;
    for (const filesystem::directory_entry& dir_entry : filesystem::directory_iterator(p)) {
        // если объект - каталог
        if (dir_entry.is_directory()) {
            directories.push_back(dir_entry);
            PrintTree(cout, dir_entry.path(), offset + 2);
        }
    }

    sort(directories.begin(), directories.end(),
        [](const filesystem::directory_entry& lhs_dir, const filesystem::directory_entry& rhs_dir) {
            return lhs_dir.path().filename().string() > rhs_dir.path().filename().string();
        });

    for (const filesystem::directory_entry& dir_entry : directories) {
        dst << string(offset, ' ') << dir_entry.path().filename().string() << '\n';
        for (const filesystem::directory_entry& file : filesystem::directory_iterator(dir_entry.path())) {
            dst << string(offset + 2, ' ') << file.path().filename().string() << '\n';
        }
    }
}

void PrintTree(ostream& dst, const path& p) {
    dst << p.filename().string() << '\n';
    PrintTree(dst, p, 2);
}

int main() {
    error_code err;
    filesystem::remove_all("test_dir", err);
    filesystem::create_directories("test_dir"_p / "a"_p, err);
    filesystem::create_directories("test_dir"_p / "b"_p, err);

    ofstream("test_dir"_p / "b"_p / "f1.txt"_p);
    ofstream("test_dir"_p / "b"_p / "f2.txt"_p);
    ofstream("test_dir"_p / "a"_p / "f3.txt"_p);

    ostringstream out;
    PrintTree(out, "test_dir"_p);
    cout << out.str();
    assert(out.str() ==
        "test_dir\n"
        "  b\n"
        "    f2.txt\n"
        "    f1.txt\n"
        "  a\n"
        "    f3.txt\n"sv
    );
}