ToCzDj - Online C++ Compiler & Debugging Tool

#include <bits/stdc++.h>
using namespace std;

long long mirrorScore(const string &s) {
    int n = s.size();
    // stacks[k] will hold unmarked indices i where s[i] == 'a'+k
    vector<vector<int>> stacks(26);
    long long score = 0;

    for (int i = 0; i < n; i++) {
        char c = s[i];
        // compute mirror letter
        char m = char('z' - (c - 'a'));
        int mi = m - 'a';

        if (!stacks[mi].empty()) {
            // found a mirror-match; use the closest j
            int j = stacks[mi].back();
            stacks[mi].pop_back();
            score += (i - j);
        } else {
            // no mirror waiting: add this index to its letter-stack
            stacks[c - 'a'].push_back(i);
        }
    }

    return score;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    string S;
    cin >> S;

    cout << mirrorScore(S) << "\n";
    return 0;
}