Submission #6417655

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Source Code Expand

# include <algorithm>
# include <iostream>
# include <iterator>
# include <vector>
# include <string>
# include <queue>
# include <stack>
# include <cmath>
# include <set>


using namespace std;
const int mod = 1e9+7;


int main() {
    int n, a, b; cin >> n >> a >> b; --a; --b;

    vector<vector<int>> dist(n);
    for (int i=0; i<n; ++i) {
        dist.at(i).resize(n);
        fill(dist.at(i).begin(), dist.at(i).end(), 1001001001);
        dist.at(i).at(i) = 0;
    }

    int m, x, y; cin >> m;
    vector<vector<int>> g(n), ng(n);  // ng: new graph

    for (int i=0; i<m; ++i) {
        cin >> x >> y; --x; --y;
        dist.at(x).at(y) = 1;
        dist.at(y).at(x) = 1;
        g[x].push_back(y);
        g[y].push_back(x);
    }

    // Floyd-Warshall
    for (int k=0; k<n; ++k) {
        for (int i=0; i<n; ++i) {
            for (int j=0; j<n; ++j) {
                dist.at(i).at(j) = min(dist.at(i).at(j), dist.at(i)[k]+dist[k].at(j));
            }
        }
    }

    /*
    for (int i=0; i<n; ++i) {
        for (int j=0; j<n; ++j) {
            cout << " " << dist.at(i).at(j);
        }
        cout << endl;
    }
    */

    int minimal_dist = dist.at(a).at(b);

    vector<bool> used(n);

    // cost, next_node
    priority_queue<pair<int, int>,
        vector<pair<int, int>>,
        greater<pair<int, int>>> edge_queue;

    fill(used.begin(), used.end(), false);
    edge_queue.push(make_pair(0, a));

    pair<int, int> edge;
    int cost, node;

    // bfs: enumerate edges
    while (!edge_queue.empty()) {
        edge = edge_queue.top(); edge_queue.pop();
        cost = edge.first; node = edge.second;
        if (used.at(node)) continue;

        if (cost == minimal_dist) continue;
        for (int next_node: g.at(node)) {
            if (used.at(next_node)) continue;
            ng.at(node).push_back(next_node);
            edge_queue.push(make_pair(cost+1, next_node));
        }

        used.at(node) = true;
    }

    vector<long long> ans(n);
    ans[a] = 1;

    // start
    fill(used.begin(), used.end(), false);
    edge_queue.push(make_pair(0, a));

    while(!edge_queue.empty()) {
        edge = edge_queue.top(); edge_queue.pop();
        cost = edge.first; node = edge.second;
        if (used.at(node)) continue;
        
        for (int next_node: ng.at(node)) {
            if (used.at(next_node)) continue;
            edge_queue.push(make_pair(cost+1, next_node));
            ans.at(next_node) += ans.at(node);
            ans.at(next_node) = ans.at(next_node) % mod;
        }

        used.at(node) = true;
    }

    cout << ans[b] << endl;
    return 0;
}

Submission Info

Judge Result