HackerRank "Prim's (MST) : Special Subtree"

An intuitive Prim algorithm impl.

#include <vector>
#include <iostream>
#include <queue>
#include <unordered_map>
#include <unordered_set>
using namespace std;

struct Edge
{
    Edge() :s(), t(), d() {}
    Edge(unsigned rs, unsigned rt, unsigned rd) :
        s(rs), t(rt), d(rd) {}

    unsigned s;
    unsigned t;
    unsigned d;

    bool operator()(const Edge &e1, const Edge &e2)
    {
        return e1.d > e2.d;
    }
};

int main()
{
    long n, m; cin >> n >> m;
    //    from -> to -> (length, id)
    unordered_map<unsigned, unordered_map<unsigned, unsigned>> g;
    for (int i = ; i < m; i++)
    {
        unsigned a, b, d; cin >> a >> b >> d;
        g[a][b] = g[b][a] = d;
    }
    unsigned s; cin >> s;

    unordered_set<unsigned> pickedSet; // linked node inx

    priority_queue<Edge, vector<Edge>, Edge> q;
    for (auto &kv : g[s])
    {
        q.push(Edge(s, kv.first, kv.second));
    }
    pickedSet.insert(s);

    unsigned long long ret = ;
    while (!q.empty())
    {
        Edge picked = q.top();
        q.pop();
        if (pickedSet.count(picked.t)) continue;

        pickedSet.insert(picked.t);

        // add new choices
        for (auto &kv : g[picked.t])
        {
            q.push(Edge(picked.t, kv.first, kv.second));
        }

        ret += picked.d;
    }
    cout << ret << endl;
    return ;
}