lessons/参考实现/05-最短路与类最短路/POJ-3255.cpp

#include <functional>
#include <iostream>
#include <limits>
#include <queue>
#include <utility>
#include <vector>
using namespace std;
struct Edge { int to; long long weight; };

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int n, m; cin >> n >> m;
    vector<vector<Edge>> g(n);
    while (m--) {
        int u, v; long long w; cin >> u >> v >> w; --u; --v;
        g[u].push_back({v, w}); g[v].push_back({u, w});
    }
    const long long inf = numeric_limits<long long>::max() / 4;
    vector<long long> first(n, inf), second(n, inf);
    priority_queue<pair<long long, int>, vector<pair<long long, int>>, greater<>> pq;
    first[0] = 0; pq.push({0, 0});
    while (!pq.empty()) {
        auto [d, u] = pq.top(); pq.pop();
        if (d > second[u]) continue;
        for (Edge e : g[u]) {
            long long candidate = d + e.weight;
            if (candidate < first[e.to]) {
                swap(candidate, first[e.to]);
                pq.push({first[e.to], e.to});
            }
            if (first[e.to] < candidate && candidate < second[e.to]) {
                second[e.to] = candidate;
                pq.push({second[e.to], e.to});
            }
        }
    }
    cout << second[n - 1] << '\n';
}