minimal substring including all letters from string

#include <iostream>
#include <unordered_map>
#include <unordered_set>

using namespace std;
int main() {
    string s;
    cin >> s;
    unordered_set<char> alf;
    for (auto el: s){
        alf.insert(el);
    }
    unordered_map<char, int> cur_alf;
    int l = 0;
    int r = 0;
    int ans = s.size();
    while (r != s.size()){
        while (r < s.size() && cur_alf.size() < alf.size()){
            ++cur_alf[s[r]];
            ++r;
        }
        while (cur_alf.size() == alf.size()){
            --cur_alf[s[l]];
            if (cur_alf[s[l]] == 0){
                cur_alf.erase(s[l]);
            }
            ++l;
        }
        ans = min(ans, r - l + 1);
    }
    cout << ans;
}
//abbac