Submission #121310

#	Time	Username	Problem	Language	Result	Execution time	Memory
121310		IOrtroiii	Alternating Current (BOI18_alternating)	C++14	100 / 100	118 ms	18040 KiB

alternating

#include <bits/stdc++.h>

using namespace std;

const int N = 200200;

int n, m;
int a[N];
vector<pair<int, int>> g[N];
int visit[N], tt;
int ans[N];
vector<int> path;

bool dfs(int u, int trg) {
   if (u == trg) {
      return true;
   }
   if (visit[u % n] == tt) {
      return false;
   }
   visit[u % n] = tt;
   for (auto e : g[u]) {
      int v, id;
      tie(v, id) = e;
      path.push_back(id);
      if (dfs(v, trg)) {
         return true;
      }
      path.pop_back();
   }
   return false;
}

bool go(int u) {
   visit[u] = ++tt;
   for (auto e : g[u]) {
      int v, id;
      tie(v, id) = e;
      if (v == u - 1) {
         continue;
      }
      path.push_back(id);
      if (dfs(v, u + n)) {
         return true;
      }
      path.pop_back();
   }
   return false;
}

int main() {
   ios_base::sync_with_stdio(false);
   cin >> n >> m;
   for (int i = 1; i <= m; ++i) {
      int l, r;
      cin >> l >> r;
      if (l > r) {
         ++a[l], --a[n + 1];
         ++a[1], --a[r + 1];
         --l, r += n;
         g[l].emplace_back(r, i);
         g[l + n].emplace_back(n + n, i);
      } else {
         ++a[l], --a[r + 1];
         l--;
         g[l].emplace_back(r, i);
         g[l + n].emplace_back(r + n, i);
      }
   }
   for (int i = 1; i <= n; ++i) {
      a[i] += a[i - 1];
      if (a[i] == 1) {
         cout << "impossible\n";
         return 0;
      }
   }
   for (int i = 1; i <= n; ++i) {
      if (a[i] > 2) {
         g[i].emplace_back(i - 1, 0);
         g[i + n].emplace_back(i - 1 + n, 0);
      }
   }
   for (int i = 0; i < n; ++i) {
      if (go(i)) {
         for (int v : path) {
            ans[v] = 1;
         }
         for (int j = 1; j <= m; ++j) {
            cout << ans[j];
         }
         return 0;
      }
   }
   cout << "impossible\n";
   return 0;
}

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