This C++ function must be solved using stacks only (no recursion). "List language(string input, int k)" must be the header for this function and a helper function is ok to use. No need to make a main(). The logic is what's needed.

List language(string input, int k). You will see a lot about lan- guages when you study Computational Models. Just think of the input string as a set of characters (formally called an alphabet). The language generated by is the set of all strings comprising of characters (symbols) in . This function generates a list of all strings in the language, whose length is at most k. Note that empty string is also part of the language. The order of the list does not matter. For example, if the input is "abc" and k = 2, then the output list has the following strings: (the empty string), a, b, c, aa, bb, cc, ab, ac, ba, be, ca, cb. You can assume that input does not have repeated characters