Parser.java

package rtb.onezero;

import java.util.ArrayList;
import java.util.List;
import java.util.function.BiFunction;
import java.util.stream.Collectors;

public final class Parser {

    private static BiFunction<List<String>, Character, List<String>> appender
            = (acc, c) -> acc.stream().map(str -> str + c).collect(Collectors.toList());

    public static List<String> parse(String phrase) {
        List<String> acc = new ArrayList<>();
        acc.add("");

        for (char c : phrase.toCharArray())
            if (c == '?') {
                List<String> append1 = appender.apply(acc, '1');
                acc = appender.apply(acc, '0');
                acc.addAll(append1);
            }
            else
                acc = appender.apply(acc, c);

        return acc;
    }
}