Submission #987550

#	Time	Username	Problem	Language	Result	Execution time	Memory
987550		PurpleCrayon	Alternating Current (BOI18_alternating)	C++17	0 / 100	16 ms	9436 KiB

alternating

#include <bits/stdc++.h>
using namespace std;
 
#define sz(v) int(v.size())
#define ar array
typedef long long ll;
const int N = 1e5+10, MOD = 1e9+7;

void no() {
    cout << "impossible\n";
    exit(0);
}

int ans[N];
vector<int> adj[N];

void dfs(int c, int b) {
    ans[c] = b;
    for (int nxt : adj[c]) {
        if (ans[nxt] == -1) dfs(nxt, b ^ 1);
        else if (ans[nxt] == b) no();
    }
}

void solve() {
    int n, m; cin >> n >> m;
    vector<pair<int, int>> v(m);
    for (int i = 0; i < m; i++) {
        int a, b; cin >> a >> b, --a, --b;
        v[i] = {a, b};
    }


    vector<int> cnt(n);

    auto add = [&](int l, int r) {
        cnt[l]++;
        if (r < n-1) cnt[r+1]--;
    };

    for (auto [l, r] : v) {
        if (l <= r) {
            add(l, r);
        } else {
            add(l, n-1);
            add(0, r);
        }
    }

    for (int i = 1; i < n; i++) cnt[i] += cnt[i-1];
    if (*min_element(cnt.begin(), cnt.end()) < 2) {
        no();
    }

    vector<int> use;
    for (int i = 0; i < n; i++) {
        if (cnt[i] == 2) {
            use.push_back(i);
        }
    }

    memset(ans, -1, sizeof(ans));
    auto solve_hard = [&]() {
        auto get_nxt = [&](int i) {
            int idx = lower_bound(use.begin(), use.end(), i) - use.begin();
            if (idx == sz(use)) return use[0];
            return use[idx];
        };

        // cerr << "use: ";
        // for (int x : use) cerr << x << ' ';
        // cerr << endl;

        vector<vector<int>> mine(n);
        for (int i = 0; i < m; i++) {
            auto [l, r] = v[i];
            // cerr << "> " << l << ' ' << r << endl;
            set<int> mark;
            if (l <= r) {
                int c = get_nxt(l);
                while (!mark.count(c) && l <= c && c <= r) {
                    // cerr << c << ' ';
                    mark.insert(c);
                    c = get_nxt((c + 1) % n);
                }
                // cerr << endl << endl;
            } else {
                int c = get_nxt(l);
                while (!mark.count(c) && (c >= l || c <= r)) {
                    // cerr << c << ' ';
                    mark.insert(c);
                    c = get_nxt((c + 1) % n);
                }
                // cerr << endl << endl;
            }
            for (int x : mark) {
                mine[x].push_back(i);
            }
        }

        for (int x : use) {
            assert(sz(mine[x]) == 2);
            int one = mine[x][0], two = mine[x][1];
            // cerr << "> " << one << ' ' << two << endl;
            adj[one].push_back(two);
            adj[two].push_back(one);
        }

        for (int i = 0; i < m; i++) if (sz(adj[i]) && ans[i] == -1) {
            dfs(i, 0);
        }
    };

    if (sz(use)) solve_hard();
    vector<int> one;
    for (int i = 0; i < m; i++) {
        if (ans[i] == 0 || ans[i] == -1) {
            one.push_back(i);
        }
    }

    auto reduce = [&]() { // clean one
        vector<vector<ar<int, 3>>> ev(2 * n);
        for (int x : one) {
            auto [l, r] = v[x];
            if ((r + 1) % n == l) {
                one = {x};
                break;
            }

            if (l <= r) {
                ev[l].push_back({x, +1, r});
                ev[r].push_back({x, -1, l});

                ev[l + n].push_back({x, +1, r + n});
                ev[r + n].push_back({x, -1, l + n});
            } else {
                ev[l].push_back({x, +1, r + n});
                ev[r + n].push_back({x, -1, l});
            }
        }

        set<int> bad;
        set<pair<int, int>> open;
        for (int i = 0; i < 2 * n; i++) {
            // cerr << "at: " << i << endl;
            for (auto [c, t, opp] : ev[i]) {
                if (t == +1) {
                    open.emplace(i, c);
                    // cerr << "add: " << c << endl;
                }
            }

            for (auto [c, t, opp] : ev[i]) {
                if (t == -1) {
                    auto it = open.find({opp, c});
                    if (it != open.begin()) {
                        // cerr << "mark: " << c << endl;
                        bad.insert(c);
                    }
                }
            }

            for (auto [c, t, opp] : ev[i]) {
                if (t == -1) {
                    // cerr << "remove: " << c << endl;
                    open.erase({opp, c});
                }
            }
        }

        vector<int> new_one;
        for (int x : one) if (!bad.count(x)) new_one.push_back(x);
        swap(one, new_one);
    };

    /*
    for (int x : one) cerr << x << ' ';
    cerr << endl;
    */
    reduce();
    /*
    for (int x : one) cerr << x << ' ';
    cerr << endl;
    */

    for (int x : one) {
        ans[x] = 0;
    }

    vector<int> two;
    for (int i = 0; i < m; i++) {
        if (ans[i] != 0) {
            ans[i] = 1;
            two.push_back(i);
        }
    }

    auto good = [&](vector<int> idx) { // check
        vector<int> cnt_check(n);

        auto add_check = [&](int l, int r) {
            cnt_check[l]++;
            if (r < n-1) cnt_check[r+1]--;
        };

        for (int x : idx) {
            auto [l, r] = v[x];
            if (l <= r) {
                add_check(l, r);
            } else {
                add_check(l, n-1);
                add_check(0, r);
            }
        }

        for (int i = 1; i < n; i++) cnt_check[i] += cnt_check[i-1];

        return *min_element(cnt_check.begin(), cnt_check.end()) > 0;
    };

    if (!good(one) || !good(two)) no();
    cout << "yes\n";
    return;
    for (int i = 0; i < m; i++) cout << ans[i];
    cout << '\n';
}
 
int main() {
    ios::sync_with_stdio(false); cin.tie(0);
    int T = 1;
    // cin >> T;
    while (T--) solve();
}

• Subtask 1: 0 / 13
• Subtask 2: 0 / 20
• Subtask 3: 0 / 22
• Subtask 4: 0 / 19
• Subtask 5: 0 / 26