zero_one_question_mark

#include <string>
#include <queue>
#include <vector>
#include <iostream>

int main(int argc, char* argv[]) {

    std::string allowed_characters{"01?"};

    if(argc <=1 ) {
        std::cout << "Provide a string comprised of 0, 1 or ?\n";
        return 1;
    }

    // validate input
    std::string input{argv[1]};
    for(const auto& c : input) {
        if(std::string::npos == allowed_characters.find(c)) {
            std::cout << "string includes invalid character: [" << c << "]\n";
            return 1;
        }
    }

    std::queue<std::string> queue;
    queue.push(input);

    std::string::size_type pos{};
    while(!queue.empty()) {
        auto str0{queue.front()};

        queue.pop();

        // find the next ?
        pos = str0.find_first_of('?', pos);
        if(std::string::npos == pos) {
           std::cout << str0 << "\n";
           continue;
        }

        auto str1{str0};

        str0[pos] = '0';
        str1[pos] = '1';

        queue.push(str0);
        queue.push(str1);
    } // while

    return 0;

} // main