142 lines
2.8 KiB
C++
142 lines
2.8 KiB
C++
/* Problem URL: https://codeforces.com/contest/118/problem/E */
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#include <bits/stdc++.h>
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using namespace std;
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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;
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cin >> n >> m;
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vvi graph(n);
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rep(i, 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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}
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vvi dfstree(n);
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V<pair<int, int>> ans;
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vi up(n);
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vi down(n);
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vi lvl(n);
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function<void(int, int)> make_tree = [&](int i, int d) {
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lvl[i] = d;
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for (auto j : graph[i]) {
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if (lvl[j] != 0) {
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if (lvl[j] >= lvl[i]) {
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ans.emplace_back(j, i);
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down[i]++;
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up[j]++;
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}
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continue;
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}
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ans.emplace_back(i, j);
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dfstree[i].push_back(j);
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make_tree(j, d + 1);
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}
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};
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make_tree(0, 1);
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vi dp(n);
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dp[0] = 1;
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function<bool(int)> check = [&](int i) {
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dp[i] += up[i];
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dp[i] -= down[i];
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for (auto j : dfstree[i]) {
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if (!check(j)) {
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return false;
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}
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dp[i] += dp[j];
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}
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return dp[i] != 0;
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};
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if (!check(0)) {
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cout << "0\n";
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return 0;
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}
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for (auto [u, v] : ans) {
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cout << u + 1 << ' ' << v + 1 << '\n';
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}
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}
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