153 lines
3.1 KiB
C++
153 lines
3.1 KiB
C++
/* Problem URL: https://codeforces.com/contest/954/problem/D */
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#include <bits/stdc++.h>
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#include <ext/pb_ds/assoc_container.hpp>
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#include <ext/pb_ds/tree_policy.hpp>
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using namespace std;
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using namespace __gnu_pbds;
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template <class T, class comp = less<>>
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using ordered_set = tree<T, null_type , comp , rb_tree_tag , tree_order_statistics_node_update>;
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#define V vector
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#define rmin(a, b) a = min(a, b)
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#define rmax(a, b) a = max(a, b)
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#define rep(i, lim) for (int i = 0; i < (lim); i++)
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#define nrep(i, s, lim) for (int i = s; i < (lim); i++)
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#define repv(i, v) for (auto &i : (v))
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#define fillv(v) for (auto &itr_ : (v)) { cin >> itr_; }
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#define sortv(v) sort(v.begin(), v.end())
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#define all(v) (v).begin(), (v).end()
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using vi = vector<int>;
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using vvi = vector<vi>;
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using vvvi = vector<vvi>;
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using vvvvi = vector<vvvi>;
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using ll = long long;
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using vl = vector<ll>;
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using vvl = vector<vl>;
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using vvvl = vector<vvl>;
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using vvvvl = vector<vvvl>;
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template<class v>
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auto operator<<(ostream &os, const vector<v> &vec)->ostream& {
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os << vec[0];
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for (size_t i = 1; i < vec.size(); i++) {
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os << ' ' << vec[i];
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}
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os << '\n';
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return os;
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}
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template<class v>
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auto operator>>(istream &is, vector<v> &vec)->istream& {
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for (auto &i : vec) {
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is >> i;
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}
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return is;
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}
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template<class v>
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auto operator<<(ostream &os, const vector<vector<v>> &vec)->ostream& {
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for (auto &i : vec) {
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os << i[0];
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for (size_t j = 1; j < i.size(); j++) {
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os << ' ' << i[j];
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}
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os << '\n';
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}
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return os;
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}
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template<class v>
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auto operator>>(istream &is, vector<vector<v>> &vec)->istream& {
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for (auto &i : vec) {
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for (auto &j : i) {
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is >> j;
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}
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}
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return is;
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}
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int main()
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{
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ios::sync_with_stdio(false);
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cin.tie(nullptr);
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int n, m, s, t;
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cin >> n >> m >> s >> t;
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s--, t--;
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vvi graph(n);
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vvi adj(n, vi(n));
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while (m--) {
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int a, b;
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cin >> a >> b;
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a--, b--;
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graph[a].push_back(b);
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graph[b].push_back(a);
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adj[a][b] = 1;
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adj[b][a] = 1;
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}
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int oo = INT32_MAX >> 1;
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auto bfs = [&](int s, vi &depth){
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depth[s] = 0;
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queue<int> q;
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q.push(s);
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while (!q.empty()) {
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auto i = q.front();
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q.pop();
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for (auto j : graph[i]) {
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if (depth[j] > depth[i] + 1) {
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depth[j] = depth[i] + 1;
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q.push(j);
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}
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}
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}
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};
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vi depth1(n, oo);
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vi depth2(n, oo);
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bfs(s, depth1);
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bfs(t, depth2);
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int minimal = depth1[t];
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V<bool> ispath(n);
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ll ans = 0;
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rep(i, n) {
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nrep(j, i + 1, n) {
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if (adj[i][j]) {
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continue;
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}
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auto check = [&](int u, int v) {
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int ne1 = min(depth1[u], depth1[v] + 1);
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int ne2 = min(depth2[u], depth2[v] + 1);
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return ne1 + ne2 >= minimal;
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};
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if (check(i, j) && check(j, i)) {
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ans++;
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}
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}
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}
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cout << ans << '\n';
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}
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