Submission #1218024

#	Time	Username	Problem	Language	Result	Execution time	Memory
1218024		asturnox	Tropical Garden (IOI11_garden)	C++20	49 / 100	5096 ms	48444 KiB

garden

#include <iostream>
#include <memory>
#include <vector>
#include <deque>
#include <set>
#include <map>
#include <algorithm>
using namespace std;

void answer(int X);

void count_routes(int N, int M, int P, int R[][2], int Q, int G[]) {
    // Build adjacency lists, but only keep up to 2 edges per node
    vector<vector<int>> node_edges(N + 1);
    for (int idx = 0; idx < M; ++idx) {
        int i = R[idx][0];
        int j = R[idx][1];
        if (node_edges[i].size() < 2) {
            node_edges[i].push_back(j);
        }
        if (node_edges[j].size() < 2) {
            node_edges[j].push_back(i);
        }
    }

    // Walk function: choose the neighbor that's not the one we came from
    auto do_walk = [&](int walk, int prev_walk) {
        if (node_edges[walk].empty()) {
            // No neighbors, same
            return walk;
        }
        int next_walk = node_edges[walk][0];
        if (node_edges[walk].size() == 2 && next_walk == prev_walk) {
            next_walk = node_edges[walk][1];
        }
        return next_walk;
    };

    map<pair<int, int>, pair<int, int>> state_to_cycle_start;
    map<pair<int, int>, shared_ptr<deque<pair<int, int>> >> state_to_path;
    map<pair<int, int>, shared_ptr<deque<pair<int, int>> >> state_to_cycle;

    // Can we, starting from `node`, after g steps end up at node P?
    auto can_from_node = [&](int node, int g) {
        auto path  = make_shared<deque<pair<int,int>>>();
        set<pair<int,int>> visited;

        int prev_walk = node;
        int walk = do_walk(prev_walk, -1);
        int next_walk = do_walk(walk, prev_walk);  // uses default prev_walk = -1

        while (g > 0) {
            g--;

            pair<int,int> state = make_pair(walk, next_walk);
            if (state_to_path.find(state) != state_to_path.end()) { // cached
                auto cached_path = state_to_path[state];
                auto cycle_start = state_to_cycle_start[state];

                // std::cout << "hit cache!" << std::endl;

                auto it = std::find(cached_path->begin(), cached_path->end(), state);
                auto it_cycle_start = std::find(cached_path->begin(), cached_path->end(), cycle_start);
                if (it != cached_path->end()) {
                    size_t pos = std::distance(cached_path->begin(), it);
                    size_t pos_cycle = std::distance(cached_path->begin(), it_cycle_start);

                    int start_pos;
                    int g_rem;
                    if (pos >= pos_cycle) { // already in cycle
                        // std::cout << "already in cycle!" << std::endl;
                        start_pos = pos;
                        g_rem = g;
                    } else if (pos + g >= pos_cycle) { // can reach cycle
                        start_pos = pos_cycle;
                        g_rem = g - (pos_cycle - pos);
                    } else { // cannot reach cycle, pos + g < is in path
                        return (*cached_path)[pos + g].first == P; // no need for modulo
                    }

                    // get cycle
                    auto cycle = state_to_cycle[cycle_start];
                    // now cycle
                    auto c_size = cycle->size();

                    return (*cycle)[(start_pos + g_rem) % c_size].first == P;
                } else {
                    // std::cout << "ERROR, not in cycle" << std::endl;
                    throw std::runtime_error("State not found in cached cycle");
                }

            }

            if (visited.find(state) == visited.end()) {
                visited.insert(state);
                path->push_back(state);

                int temp = walk;
                walk = do_walk(walk, prev_walk);
                if (temp == walk) {
                    return walk == P; // we are stuck at walk
                }

                prev_walk = temp;
                next_walk = do_walk(walk, prev_walk);
            } else {
                // std::cout << "hit cycle!" << std::endl;
                // std::cout << "g remaining " << g << std::endl;
                // std::cout << "path: " << std::endl;
                for (int i = 0; i < path->size(); i++) {
                    // std::cout << (*path)[i].first << " " << (*path)[i].second << std::endl;
                }

                // std::cout << "state: " << state.first << " " << state.second << std::endl;
                
                auto cycle = make_shared<deque<pair<int, int>>>(*path);
                while (cycle->front() != state) {
                    // std::cout << "popping " << cycle.front().first << " " << cycle.front().second << std::endl;
                    cycle->pop_front();
                }
                // now have a cycle

                for (auto state_it = path->begin(); state_it != path->end(); ++state_it) {
                    // cache
                    state_to_path.insert({*state_it, path});
                    state_to_cycle_start.insert({*state_it, state});
                    state_to_cycle.insert({*state_it, cycle});
                }
                
                int c = cycle->size();
                // std::cout << "cycle length " << c << std::endl;
                return (*cycle)[g % c].first == P;
            }
        }

        // std::cout << "found path:" << '\n';
        for (int i = 0; i < path->size(); i ++) {
            // std::cout << (*path)[i].first << '\n';
        }
        // std::cout << "--------" << '\n';

        // After g steps, are we at P?
        return path->back().first == P;
    };

    for (int i = 0; i < Q; i++) {
        int g_val = G[i];
        int count = 0;
        // std::cout << "i " << i << std::endl;
        for (int node = 0; node < N; ++node) {
            // std::cout << "can from node: " << node << " " << g_val << std::endl;
            bool can = can_from_node(node, g_val);
            // std::cout << "can from node " << node << " " << g_val << ": " << can << std::endl;
            if (can) {
                count++;
            }
            // std::cout << "--------------------" << std::endl;
        }
        answer(count);


        // std::cout << "----------------------------------------" << std::endl;
    }
}

• Subtask 1: 49 / 49
• Subtask 2: 0 / 20
• Subtask 3: 0 / 31