In this exercise, you will need the itemAt method from the previous programming project. Write a program that prints all strings with at most n letters, where the letters are chosen from a range first ... last of characters. The following is an outline for an algorithm to do this using a stack. Your program should use a stack to implement this algorithm:

// Writing all strings of 1 to n letters: Push first onto the stack. while (the stack is not empty) { Print all of the stack (using itemAt). if (the stack contains fewer than n letters) Push first onto the stack. else { Pop characters off the stack until the stack is empty or there is a character oth- er than last on the top. (Note: If the top character is not last, then nothing is popped off the stack.) if (the stack is not empty) Pop a character c off the stack and push c+1 (i.e., the next letter) onto the stack. } }