remove k digits

#include <iostream>

using namespace std;

class Solution {
private:
    void add_digit(string &res, char &ch){
        if (!(ch == '0' && res.size() == 0))
            res += ch;
    }

public:
    string removeKdigits(string num, int k) {
        string result = "";
        char last_char = '?';
        if (num.size() == k) return "0";

        for (char current_char : num) {
            if (k == 0){
                if (last_char != '?') {
                    add_digit(result, last_char);
                    last_char = '?';
                }
                add_digit(result, current_char);
            }
            else {
                if (last_char == '?') {
                    last_char = current_char;
                    continue;
                }

                if (current_char >= last_char) {
                    add_digit(result, last_char);
                } else {
                    k--;
                }
                last_char = current_char;
            }
        }

        if (last_char != '?')
            result += last_char;

        for (long pos = result.size() - 1; pos >= 0; pos--){
            if (k == 0) break;
            if (result[pos] == '0')
                continue;
            result.erase(pos, 1);
            k--;
        }

        if (result == "")
            result = "0";
        return result;
    }
};

int main(int argc, const char * argv[]) {
    Solution sol;
    cout << sol.removeKdigits("1234506789", 9) << endl;

    return 0;
}