$O(n26^2)$过了。

#include <bits/stdc++.h>

void Freopen() {
    freopen("", "r", stdin);
    freopen("", "w", stdout);
}

using namespace std;
const int N = 1e6 + 10, M = 2e5 + 10, inf = 1e9, mod = 998244353;

int n;

char s[N];

int sum[26][N], lst[26];

signed main() {
    ios :: sync_with_stdio(false);
    cin.tie(0), cout.tie(0);
    
    cin >> n >> (s + 1);

    for ( int i = 1; i <= n; i ++) {
        lst[s[i] - 'a'] = i;

        for ( int j = 0; j < 26; j ++) {
            if (j == s[i] - 'a') sum[j][i] = sum[j][i - 1] + 1;
            else sum[j][i] = sum[j][i - 1];
        }
    }

    int ans = 0;

    for ( register int a = 0; a < 26; a ++)
        for ( register int b = 0; b < 26; b ++) {
            if (a == b) continue  ;

            int mx = 0, id = 0;

            for ( register int i = 1; i <= n; i ++) {
                if (sum[b][i] - sum[b][id] != 0)
                    ans = max(ans, sum[a][i] - sum[b][i] + mx);

                if (mx < sum[b][i] - sum[a][i] && i != lst[b])
                    mx = sum[b][i] - sum[a][i], id = i;
            }
        }


    cout << ans;

    return 0;
}