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string string_compression(const string& s) {
    if(s.empty()) return "";
    
    unordered_map<char, int> hash;
    for(char c : s) hash[c]++;
    
    string res;
    for(auto p : hash) {
        if(p.second == 1)
            res += p.first;
        else
            res += p.first + to_string(p.second);
    }
    return res;
}

string string_compression2(const string& s) {
    char last_char = s[0];
    string res;
    int i = 1; // start from second
    int count = 1;
    while(i < s.size()) {
        if(s[i] == last_char) 
            count++;
        else {
            res += last_char + to_string(count);
            last_char = s[i];
            count = 1;
        }
        i++;
    }
    res += last_char + to_string(count); // cannot forget
    return res;
}