POI Task Letters

#include <iostream>
#include <vector>

using namespace std;

void update(vector<int> &fenwick, int index, int value) {
    while (index < fenwick.size()) {
        fenwick[index] += value;
        index += index & -index;
    }
}
int getSum(const vector<int> &fenwick, int index) {
    int sum = 0;
    while (index) {
        sum += fenwick[index];
        index -= index & -index;
    }
    return sum;
}

int main() {
    int n; cin >> n;
    string s, t; cin >> s >> t;
    vector<int> positions[26];
    for (int i = n-1; i >= 0; --i) positions[s[i]-'A'].push_back(i);

    vector<int> fenwick(1+n);

    long long moves = 0;
    for (int i = 0; i < n; ++i) {
        int cur = positions[t[i]-'A'].back();
        positions[t[i]-'A'].pop_back();
        moves = moves + (cur+getSum(fenwick, n-cur))-i;
        update(fenwick, n-cur, +1);
    }
    cout << moves << endl;
    return 0;
}