Submission #1065928

#	Time	Username	Problem	Language	Result	Execution time	Memory
1065928		Shadow1	Cryptography (NOI20_crypto)	C++17	100 / 100	298 ms	31684 KiB

Crypto

// Programmer: Shadow1

#include <bits/stdc++.h>
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
using namespace std;

using ll = long long;
using ld = long double;
using ull = unsigned long long;
using str = string; // yay python!

#define i64 int64_t
#define show(x) cerr << (#x) << " = " << (x) << '\n';
#define output_vector(v) for(auto &x : v){cout << x << ' ';}cout << '\n';
#define output_pairvector(v) for(auto &x : v){cout << x.first << " " << x.second << '\n';}
#define vt vector
#define pq priority_queue
#define pb push_back
#define eb emplace_back
#define pii pair<int,int>
#define umap unordered_map
#define uset unordered_set
#define fir first
#define sec second
#define sz(x) ll(x.size())
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define int ll
#define discretize(x) sort(x.begin(), x.end()); x.erase(unique(x.begin(), x.end()), x.end());

// T: O(n^3)
// M : O(n + k log n) 

const int mxN = 50005, INF = 2e9;

vector<int64_t> ft;

int lsb(int x) {
	return (x & (-x));
}

void build(const vector<int64_t> &f) {  // Build the fenwick tree array from the frequency array
	int m = sz(f) - 1;
	ft.resize(m+1);
	for(int i=1; i<=m; ++i) {
		ft[i] += f[i];
		if(i + lsb(i) <= m)
			ft[i+lsb(i)] += ft[i];
	}
}

int64_t query(int i, int j) {  // i <= j   // O(log m)
	int64_t sum = 0;
	while(j > 0) {
		sum += ft[j];
		j -= lsb(j);
	}
	--i;
	while(i > 0) {
		sum -= ft[i];
		i -= lsb(i);
	}
	return sum;
}

void update(int i, int64_t v) {
	for(; i<sz(ft); i+=lsb(i))   // O(log m)
		ft[i] += v;
}

const int m = 1e9 + 7;
void solve() {
	int n;
	cin >> n;
	vector<int> a(n+1), fact(n+1), s(n+1);
	map<int,int> r;
	for(int i=1; i<=n; ++i) {
		cin >> a[i];
		s[i] = a[i];
	}
	sort(all(s));
	for(int i=1; i<=n; ++i)
		r[s[i]] = i;
	fact[0] = 1;
	for(int i=1; i<=n; ++i) {
		fact[i] = fact[i-1] * i;
		fact[i] %= m;
	}
	ft.assign(n+2, 0);
	update(r[a[n]], 1);
	int ans = 1;
	for(int i=n-1; i; --i) {
		int cnt = query(1, r[a[i]]-1);
		ans += (cnt * fact[n-i]) % m;
		ans %= m;
		update(r[a[i]], 1);
	}
	cout << ans << '\n';
}
	


signed main() {
	// freopen("output.txt", "w", stdout);
	// freopen("input.txt", "r", stdin);
	ios::sync_with_stdio(false);
	cin.tie(NULL); 
	int T = 1;
	// cin >> T;

	while(T--)
		solve();
	return 0;
}

/* CHECK :
1. COMPARATOR FUNCTION MUST RETURN FALSE WHEN ARGUMENTS ARE EQUAL!!!
2. Overflow! Typecase int64_t on operations if varaibles are int
3. Check array bounds!!!
4. Check array indexing!!!
5. Edge cases. (N==1)!!!
*/

• Subtask 1: 5 / 5
• Subtask 2: 9 / 9
• Subtask 3: 10 / 10
• Subtask 4: 11 / 11
• Subtask 5: 21 / 21
• Subtask 6: 13 / 13
• Subtask 7: 19 / 19
• Subtask 8: 12 / 12