1
auto cmp = [&](const Edge& a, const Edge& b) {
    return a.cost > b.cost;   // 小根堆：cost 小的优先
};

priority_queue<Edge, vector<Edge>, decltype(cmp)> pq(cmp);

2
struct Compare {
    bool operator()(const Edge& a, const Edge& b) const {
        return a.cost > b.cost;   // 小根堆
    }
};

priority_queue<Edge, vector<Edge>, Compare> pq;

3
using F = function<bool(const Edge&, const Edge&)>;
priority_queue<Edge, vector<Edge>, F> pq(
    [](const Edge& a, const Edge& b){
        return a.cost > b.cost;  // 小根堆
    }
);

4
struct Edge{
    int from, to, cost;
    Edge(int f, int t, int c): from(f), to(t), cost(c) {}

    bool operator<(const Edge& other) const {
        return cost > other.cost;  // 小根堆
    }
    //一定是<，不能是>
}