Submission #1243856

#	Time	Username	Problem	Language	Result	Execution time	Memory
1243856		longgggg	Commuter Pass (JOI18_commuter_pass)	C++20	15 / 100	2096 ms	31988 KiB

commuter_pass

#include <bits/stdc++.h>
using namespace std;
// ===== BITWISE ===== //
#define MASK(i) (1LL << (i))
#define BIT(x, i) (1LL & ((x) >> (i)))
#define ON(x, i) ((x) | MASK(i))
#define OFF(x, i) ((x) & ~MASK(i))
#define LASTBIT(mask) ((mask) & -(mask))
// ===== OTHER ===== //
#define Longgggg ios_base::sync_with_stdio(0); cin.tie(0);
#define FOR(i, a, b) for (int i = (a); i <= (b); i++)
#define FORD(i, a, b) for (int i = (a); i >= (b); i--)
#define mod(x, k) ((((x) % (k)) + (k)) % (k))
#define all(x) begin(x), end(x)
#define fi first
#define se second
#define ll long long
#define endl '\n'
// ===== FILE ===== //
#define IN "A.in"
#define OUT "A.out"
#define DEBUG "debug.out"
//==================//

const ll INF = 1e18;
const int mxN = 1e5 + 5;

int N, M, S, T, U, V;
vector<pair<int, int>> adj[mxN];
vector<tuple<int, int, int>> edges;

vector<ll> dijkstra(int start) {
    vector<ll> dist(N + 1, INF);
    priority_queue<pair<ll, int>, vector<pair<ll, int>>, greater<>> pq;
    dist[start] = 0;
    pq.emplace(0, start);
    while (!pq.empty()) {
        auto [d, u] = pq.top(); pq.pop();
        if (d > dist[u]) continue;
        for (auto &[v, cost] : adj[u]) {
            if (dist[v] > dist[u] + cost) {
                dist[v] = dist[u] + cost;
                pq.emplace(dist[v], v);
            }
        }
    }
    return dist;
}

ll run_dijkstra_with_pass(set<pair<int, int>> &pass_edges) {
    vector<ll> dist(N + 1, INF);
    priority_queue<pair<ll, int>, vector<pair<ll, int>>, greater<>> pq;
    dist[U] = 0;
    pq.emplace(0, U);
    while (!pq.empty()) {
        auto [d, u] = pq.top(); pq.pop();
        if (d > dist[u]) continue;
        for (auto &[v, cost] : adj[u]) {
            int realCost = pass_edges.count({u, v}) ? 0 : cost;
            if (dist[v] > dist[u] + realCost) {
                dist[v] = dist[u] + realCost;
                pq.emplace(dist[v], v);
            }
        }
    }
    return dist[V];
}

void solve() {
    cin >> N >> M;
    cin >> S >> T;
    cin >> U >> V;

    FOR(i, 1, M) {
        int a, b, c;
        cin >> a >> b >> c;
        adj[a].emplace_back(b, c);
        adj[b].emplace_back(a, c);
        edges.emplace_back(a, b, c);
    }

    auto distS = dijkstra(S);
    auto distT = dijkstra(T);
    ll distST = distS[T];

    // Build subgraph with only shortest-path edges
    vector<vector<pair<int, int>>> spG(N + 1);
    for (auto &[u, v, c] : edges) {
        if (distS[u] + c + distT[v] == distST)
            spG[u].emplace_back(v, c);
        if (distS[v] + c + distT[u] == distST)
            spG[v].emplace_back(u, c);
    }

    // Use DFS to trace multiple shortest paths from S → T
    const int LIMIT_PATHS = 500;
    vector<vector<int>> paths;
    vector<int> path;

    function<void(int)> dfs = [&](int u) {
        if ((int)paths.size() >= LIMIT_PATHS) return;
        path.push_back(u);
        if (u == T) {
            paths.push_back(path);
            path.pop_back();
            return;
        }
        for (auto &[v, _] : spG[u]) {
            dfs(v);
        }
        path.pop_back();
    };

    dfs(S);

    ll best = INF;
    for (auto &p : paths) {
        set<pair<int, int>> pass_edges;
        for (int i = 0; i + 1 < (int)p.size(); i++) {
            pass_edges.insert({p[i], p[i + 1]});
            pass_edges.insert({p[i + 1], p[i]});  // bidirectional
        }
        best = min(best, run_dijkstra_with_pass(pass_edges));
    }

    // Also test no-pass case
    auto distU = dijkstra(U);
    best = min(best, distU[V]);

    cout << best << endl;
}

signed main() {
    if (fopen(IN, "r")) {
        freopen(IN, "r", stdin);
        freopen(OUT, "w", stdout);
        freopen(DEBUG, "w", stderr);
    }
    Longgggg

    solve();
    return 0;
}

Compilation message (stderr)

commuter_pass.cpp: In function 'int main()':
commuter_pass.cpp:135:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
  135 |         freopen(IN, "r", stdin);
      |         ~~~~~~~^~~~~~~~~~~~~~~~
commuter_pass.cpp:136:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
  136 |         freopen(OUT, "w", stdout);
      |         ~~~~~~~^~~~~~~~~~~~~~~~~~
commuter_pass.cpp:137:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
  137 |         freopen(DEBUG, "w", stderr);
      |         ~~~~~~~^~~~~~~~~~~~~~~~~~~~

• Subtask 1: 0 / 16
• Subtask 2: 15 / 15
• Subtask 3: 0 / 24
• Subtask 4: 0 / 45