String And Its Frequency

• Time : $O(n)$
• Space : $O(1)$

string Solution::solve(string A) {
    unordered_map<char,int> freq;
    for(auto a : A) freq[a]++;
    string res = "";
    for(auto a : A) {
        if(freq.count(a)) {
            res.push_back(a);
            res += to_string(freq[a]);
            freq.erase(a);
        }
    }
    return res;
}