If we have a string <code>s</code> of length <code>x</code>, we can generate all strings of length <code>x + 1</code>.

If <code>s</code> has 0 as the last character, we can only append 1, whereas if the last character is 1, we can append both 0 and 1.

We can use recursion and backtracking to generate all such strings.