How to print undirected graph like LeetCode's serialisation? about leetcode HOT 1 CLOSED

#include <iostream>
#include <unordered_map>
#include <unordered_set>
#include <algorithm>
#include <deque>
#include <string>
#include <sstream>

void printUndirectedGraph(UndirectedGraphNode *node) {
    std::ostringstream out;
    std::unordered_map<int, std::unordered_set<int>> map{{0, std::unordered_set<int>()}};
    std::deque<UndirectedGraphNode*> cache;
    cache.push_back(node);
    do {
        std::sort(cache.begin(), cache.end(),[](UndirectedGraphNode* n1, UndirectedGraphNode* n2){
            return n1->label < n2->label;
        });
        UndirectedGraphNode *node = cache.front(); cache.pop_front();
        out << node->label;
        auto found = map.find(node->label);
        for (UndirectedGraphNode* neighbor : node->neighbors) {
            if (map.find(neighbor->label) == map.end()) cache.push_back(neighbor);
            if (found != map.end()) {
                std::unordered_set<int> &exist = found->second;
                if (exist.find(neighbor->label) == exist.end()) out << "," << neighbor->label;
            }
            map[neighbor->label].emplace(node->label);
        }
        out << "#";
    } while (!cache.empty());
    std::string out_str = out.str();
    std::cout << out_str.substr(0, out_str.size()-1) << std::endl;
}