#include <iostream>
#include <unordered_set>
#include <string>
using namespace std;
 
int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
 
    int t;
    cin >> t;
 
    while (t--) {
        int n;
        cin >> n;
        string s;
        cin >> s;
 
        unordered_set<char> all_chars(s.begin(), s.end());
        unordered_set<char> left_chars;
        int max_f = 0;
 
        for (int i = 0; i < n - 1; ++i) {
            left_chars.insert(s[i]);
            all_chars.erase(s[i]);
            max_f = max(max_f, (int)left_chars.size() + (int)all_chars.size());
        }
 
        cout << max_f << "\n";
    }
 
    return 0;
}

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