Submission #945862

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Source Code Expand

#include <bits/stdc++.h>
#include <ext/pb_ds/detail/standard_policies.hpp>
#include <ext/pb_ds/assoc_container.hpp> // Общий файл. 
#include <ext/pb_ds/tree_policy.hpp> // Содержит класс tree_order_statistics_node_update
using namespace std;
using namespace __gnu_pbds;

typedef tree<int,null_type,less<int>,rb_tree_tag,tree_order_statistics_node_update> ordered_set;

using ll = long long;
using ull = unsigned long long;
using ld = long double;

#define forn(i, a, n) for (int i = a; i < n; ++i)
#define ford(i, a, n) for (int i = n - 1; i >= a; --i)
#define fore(i, a, n) for (int i = a; i <= n; ++i)
#define all(a) (a).begin(), (a).end()
#define fs first
#define sn second
#define trace(a)\
    for (auto i : a) cerr << i << ' ';\
    cerr << '\n'
#define eb emplace_back

#ifndef M_PI
const ld M_PI = acos(-1.0);
#endif

const ld eps = 1e-9;
const int INF = 2000000000;
const ll LINF = 1ll * INF * INF;
const ll MOD = 1000000007;

ll Pow(ll a, ll b) {
    ll x = 1, y = a;
    while (b) {
        if (b % 2) {
            x = x * y % MOD;
        }
        y = y * y % MOD;
        b /= 2;
    }
    return x;
}

ll Inv(ll x) {
    return Pow(x, MOD - 2);
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n;
    cin >> n;
    vector<ll> fact(n + 1);
    fact[0] = 1;
    fore(i, 1, n) fact[i] = fact[i - 1] * i % MOD;
    vector<int> p(n);
    ordered_set frees;
    forn(i, 0, n) frees.insert(i);
    forn(i, 0, n) {
        cin >> p[i];
        --p[i];
        if (p[i] != -1)
            frees.erase(p[i]);
    }
    ordered_set ps;
    int free = 0;
    forn(i, 0, n) if (p[i] == -1) ++free;
    ll ans = 0;
    ll freeAfter = 0;
    ll avg = 0;
    ford(i, 0, n) {
        if (p[i] == -1) {
            ll cnt = avg * Inv(free) % MOD;
            cnt += freeAfter * Inv(2);
            cnt %= MOD;
            ans = (ans + cnt * fact[free] % MOD * fact[n - 1 - i]) % MOD;
            ++freeAfter;
        } else {
            ll cnt = ps.order_of_key(p[i]);
            ps.insert(p[i]);
            ll add = freeAfter * frees.order_of_key(p[i]) % MOD * Inv(free) % MOD;
            ll rez = (cnt + add) * fact[free] % MOD * fact[n - 1 - i];
            ans = (ans + rez) % MOD;
            avg += free - frees.order_of_key(p[i]);
            if (avg >= MOD) avg -= MOD;
        }
    }
    ans += fact[free];
    ans %= MOD;
    cout << ans << '\n';
}

Submission Info

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