#include <bits/stdc++.h> /* Chang ki si xuyen man dem Di lac vao trong giac mong */ using namespace std; using ll = long long; #define int long long #define pii pair<ll, ll> #define fi first #define se second const ll N = 5e5 + 5, inf = 1e18, mod = 1e9 + 7, block = 320, lim = 19; int n; string s; int a[N]; vector <int> pos[30]; set <int> pos1[30]; int b[N]; vector <int> tmp, tmp1; int c[N]; struct Fenwick { vector <int> bit; int sz; Fenwick(int _n) { bit.assign(_n + 5, 0); sz = _n; } void update(int i, int val) { while(i <= sz) { bit[i] += val; i += i & -i; } } int get(int i) { int res = 0; while(i > 0) { res += bit[i]; i -= i & -i; } return res; } } fw(N); signed main() { ios::sync_with_stdio(false); cin.tie(0); if (fopen(".inp", "r")) { freopen(".inp", "r", stdin); freopen(".out", "w", stdout); } cin >> n >> s; for (int i = 1; i <= 2 * n; i++) { a[i] = s[i - 1] - 'a'; pos[a[i]].push_back(i); } for (int i = 0; i < 26; i++) { int have = pos[i].size() / 2; for (int j = 0; j < pos[i].size(); j++) { if (j < have) { b[pos[i][j]] = 1; } else { b[pos[i][j]] = 2; } } } int ans = 0; for (int i = 1, cnt = 0; i <= 2 * n; i++) { if (b[i] == 1) { ans += cnt; tmp.push_back(a[i]); } else { cnt++; tmp1.push_back(a[i]); } } for (int i = 0; i < tmp1.size(); i++) { pos1[tmp1[i]].insert(i + 1); } // cout << tmp.size() << ' ' << n << '\n'; for (int i = 0; i < tmp.size(); i++) { int x = *pos1[tmp[i]].begin(); pos1[tmp[i]].erase(pos1[tmp[i]].begin()); c[i + 1] = x; } // for (int i = 1; i <= n; i++) cout << c[i] << ' '; vector <int> tt(n + 5, 0); for (int i = 1; i <= n; i++) tt[c[i]] = i; for (int i = 1; i <= n; i++) { ans += fw.get(n - tt[i] + 1); fw.update(n - tt[i] + 1, 1); } cout << ans << '\n'; return 0; }

Main.cpp: In function 'int main()':
Main.cpp:52:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   52 |         freopen(".inp", "r", stdin);
      |         ~~~~~~~^~~~~~~~~~~~~~~~~~~~
Main.cpp:53:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   53 |         freopen(".out", "w", stdout);
      |         ~~~~~~~^~~~~~~~~~~~~~~~~~~~~