Submission #629485

#	Time	Username	Problem	Language	Result	Execution time	Memory
629485		abeker	Thousands Islands (IOI22_islands)	C++17	29 / 100	212 ms	23612 KiB

islands

#include <bits/stdc++.h>
#include "islands.h"
using namespace std;

typedef pair <vector <int>, vector <int>> pvv;

bool check(int M, vector <int> u, vector <int> v, vector <int> s) {
  int curr = 0, prev = -1;
  vector <int> occ(M);
  for (auto it : s) {
    //if (it == prev)
    //  return false;
    if (u[it] != curr)
      return false;
    curr = v[it];
    swap(u[it], v[it]);
    occ[it] ^= 1;
    prev = it;
  }
  return !curr;
  //return !curr && !count(occ.begin(), occ.end(), 1);
}

vector <int> min_rot(vector <int> v) {
  rotate(v.begin(), min_element(v.begin(), v.end()), v.end());
  return v;
}

variant <bool, vector <int>> find_journey(int N, int M, vector <int> u, vector <int> v) {
  queue <int> sinks;
  vector <unordered_set <int>> in(N), out(N);
  for (int i = 0; i < M; i++) {
    out[u[i]].insert(i);
    in[v[i]].insert(i);
  }
  auto refresh = [&](int x) {
    if (out[x].empty())
      sinks.push(x);
  };
  for (int i = 0; i < N; i++)
    refresh(i);
  int curr = 0;
  vector <int> sofar;
  while (1) {
    auto other = [&](int e, int x) {
      return u[e] ^ v[e] ^ x;
    };
    auto remove = [&](int x) {
      for (auto it : in[x]) {
        int tmp = other(it, x);
        out[tmp].erase(it);
        refresh(tmp);
      }
      for (auto it : out[x])
        in[other(it, x)].erase(it);
    };
    for (; !sinks.empty(); sinks.pop()) {
      if (sinks.front() == curr)
        return false;
      remove(sinks.front());
    }
    if (out[curr].size() == 1) {
      int edge = *out[curr].begin();
      remove(curr);
      curr = other(edge, curr);
      sofar.push_back(edge);
    }
    else {
      auto find_cycle = [&](int x, int e, bool b) {
        int y = x;
        vector <int> path = {e};
        vector <bool> bio(N);
        bio[x] = b;
        for (x = other(e, x); !bio[x]; x = other(e, x)) {
          path.push_back(e = *out[x].begin());
          bio[x] = true;
        }
        bool flag = false;
        vector <int> tail, cycle;
        for (auto it : path) {
          flag |= y == x;
          if (flag)
            cycle.push_back(it);
          else
            tail.push_back(it);
          y = other(it, y);
        }
        return pvv(tail, cycle);
      };
      auto it = out[curr].begin();
      pvv cyc1 = find_cycle(curr, *it++, true);
      pvv cyc2 = find_cycle(curr, *it++, false);
      vector <int> sol;
      auto append = [&](vector <int> &v) {
        sol.insert(sol.end(), v.begin(), v.end());
        reverse(v.begin(), v.end());
      };
      append(sofar);
      bool eq = min_rot(cyc1.second) == min_rot(cyc2.second);
      for (int i = 0; i < 2; i++) {
        auto traverse = [&](pvv &p1, pvv &p2) {
          append(p1.first);
          append(p1.second);
          if (eq)
            reverse(p2.second.begin(), p2.second.end());
          append(p1.first);
        };
        traverse(cyc1, cyc2);
        traverse(cyc2, cyc1);
      }
      append(sofar);
      assert(check(M, u, v, sol));
      return sol;
    }
  }
}

Compilation message (stderr)

islands.cpp: In function 'bool check(int, std::vector<int>, std::vector<int>, std::vector<int>)':
islands.cpp:8:17: warning: variable 'prev' set but not used [-Wunused-but-set-variable]
    8 |   int curr = 0, prev = -1;
      |                 ^~~~

• Subtask 1: 5 / 5
• Subtask 2: 0 / 5
• Subtask 3: 0 / 21
• Subtask 4: 24 / 24
• Subtask 5: 0 / 45