Число которое встречается меньше всего

#include <iostream>
#include <iomanip>
#include <string>
#include <optional>
#include <map>
#include <algorithm>
 
auto get_rariest_digit(const std::string& s)
{
    std::map<char, unsigned> duplication_map;
    std::for_each(s.begin(), s.end(), [&duplication_map, &s](unsigned char c) { std::isdigit(c) ? duplication_map[c]++ : 0; });
    if (duplication_map.empty()) return std::optional<char>();
    auto rariest = std::min_element(duplication_map.begin(), duplication_map.end(), [](auto& e1, auto& e2) { return e1.second < e2.second; });
    return std::optional<char>(rariest->first);
}
 
int main()
{
    std::string s1 = "this line contains no digits at all";
    auto rariest = get_rariest_digit(s1);
    if (rariest.has_value()) std::cout << "The rariest digit is " << rariest.value() << std::endl;
    else std::cout << "The original line contains no digits!" << std::endl;
 
    std::string s2 = "this line contains digits: 1234123121";
    rariest = get_rariest_digit(s2);
    if (rariest.has_value()) std::cout << "The rariest digit is " << rariest.value() << std::endl;
    else std::cout << "The original line contains no digits!" << std::endl;
 
    return 0;
}