Initial commit: C++ courseware

2.Course/2.深入浅出程序设计竞赛（提高篇）/最短路/C++源码/03_flights.cpp

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+// P4568 / 教材例 10-2：隐式分层图，最多 k 次免费。
+// n m k；s t；m 行 u v w。城市从 0 编号，无向非负边。
+// 课堂扩展约定：如果没有路线，输出 -1；原题未指定此情况的输出。
+#include <iostream>
+#include <vector>
+#include <queue>
+#include <tuple>
+#include <functional>
+#include <algorithm>
+using namespace std;
+using ll = long long;
+using State = tuple<ll, int, int>;
+struct Edge { int to; ll w; };
+int main() {
+    ios::sync_with_stdio(false); cin.tie(nullptr);
+    int n, m, k, s, t;
+    if (!(cin >> n >> m >> k >> s >> t)) return 0;
+    vector<vector<Edge>> g(n);
+    for (int i = 0; i < m; ++i) {
+        int u, v; ll w; cin >> u >> v >> w;
+        g[u].push_back({v, w});
+        g[v].push_back({u, w});
+    }
+    const ll INF = (1LL << 60);
+    vector<vector<ll>> dis(k + 1, vector<ll>(n, INF));
+    priority_queue<State, vector<State>, greater<State>> q;
+    dis[0][s] = 0;
+    q.push({0, s, 0});
+    while (!q.empty()) {
+        State cur = q.top(); q.pop();
+        ll d = get<0>(cur);
+        int u = get<1>(cur), j = get<2>(cur);
+        if (d != dis[j][u]) continue;
+        for (const auto &e : g[u]) {
+            int v = e.to;
+            ll nd = d + e.w;
+            if (nd < dis[j][v]) {
+                dis[j][v] = nd;
+                q.push({nd, v, j});
+            }
+            if (j < k && d < dis[j + 1][v]) {
+                dis[j + 1][v] = d;
+                q.push({d, v, j + 1});
+            }
+        }
+    }
+    ll ans = INF;
+    for (int j = 0; j <= k; ++j) ans = min(ans, dis[j][t]);
+    cout << (ans == INF ? -1 : ans) << '\n';
+}