Submission #831807

#	Time	Username	Problem	Language	Result	Execution time	Memory
831807		finn__	Ancient Books (IOI17_books)	C++17	100 / 100	1471 ms	69708 KiB

books

#include "books.h"
#include <bits/stdc++.h>
using namespace std;
using L = long long;

long long minimum_walk(std::vector<int> p, int s)
{
    int const n = p.size();
    L ans = 0;
    for (int i = 0; i < n; ++i)
        ans += abs(i - p[i]);

    vector<pair<int, int>> cycle(n, {-1, -1});
    for (int i = 0; i < n; ++i)
        if (cycle[i].first == -1)
        {
            int x = i, a = n, b = -1;
            do
            {
                a = min(a, x);
                b = max(b, x);
                x = p[x];
            } while (x != i);
            do
            {
                cycle[x] = {a, b};
                x = p[x];
            } while (x != i);
        }

    set<int> unexplored;
    for (int i = 0; i < n; ++i)
        if (p[i] != i)
            unexplored.insert(i);
    int a = s, b = s;

    auto explore_cycle = [&](int u)
    {
        auto it = unexplored.find(u);
        while (it != unexplored.end())
        {
            a = min(a, u);
            b = max(b, u);
            unexplored.erase(it);
            u = p[u];
            it = unexplored.find(u);
        }
    };

    explore_cycle(s);

    int first_non_id = 0, last_non_id = n - 1;
    while (first_non_id < s && p[first_non_id] == first_non_id)
        ++first_non_id;
    while (last_non_id > s && p[last_non_id] == last_non_id)
        --last_non_id;

    while (!unexplored.empty())
    {
        auto it = unexplored.lower_bound(a);
        if (it != unexplored.end() && *it <= b)
        {
            explore_cycle(*it);
        }
        else
        {
            int cost_left = 0, cost_right = 0, component_end = s;

            int i = a;
            while (i >= first_non_id)
            {
                if (cycle[i].second > b)
                    break;
                component_end = min(component_end, min(i, p[i]));
                if (i == component_end && i != first_non_id)
                    cost_left += 2;
                --i;
            }

            int j = i;

            i = b;
            component_end = s;
            while (i <= last_non_id)
            {
                if (cycle[i].first < a)
                    break;
                component_end = max(component_end, max(i, p[i]));
                if (i == component_end && i != last_non_id)
                    cost_right += 2;
                ++i;
            }

            if (i == last_non_id + 1)
                return ans + cost_left + cost_right;
            if (cost_left < cost_right)
            {
                ans += cost_left;
                a = j;
            }
            else
            {
                ans += cost_right;
                b = i;
            }
        }
    }

    return ans;
}

• Subtask 1: 12 / 12
• Subtask 2: 10 / 10
• Subtask 3: 28 / 28
• Subtask 4: 20 / 20
• Subtask 5: 30 / 30