Question

Full disclosure: this is for an assignment, so please don’t post actual code solutions!

I have an assignment that requires me to take a string from the user and pass it into a stack and a queue, then use those two to compare the chars to determine if the string is a palindrome. I have the program written, but there appears to be some logic error somewhere. Here’s the relevant code:

public static void main(String[] args) {

    UserInterface ui = new UserInterface();
    Stack stack = new Stack();
    Queue queue = new Queue();
    String cleaned = new String();
    boolean palindrome = true;

    ui.setString("Please give me a palindrome.");
    cleaned = ui.cleanString(ui.getString());

    for (int i = 0; i < cleaned.length(); ++i) {
        stack.push(cleaned.charAt(i));
        queue.enqueue(cleaned.charAt(i));
    }

    while (!stack.isEmpty() && !queue.isEmpty()) {
        if (stack.pop() != queue.dequeue()) {
            palindrome = false;
        }
    }

    if (palindrome) {
        System.out.printf("%s is a palindrome!", ui.getString());
    } else
        System.out.printf("%s is not a palindrome :(", ui.getString());

    stack.dump();
    queue.clear();

}

 public class Stack {

   public void push(char c) {
    c = Character.toUpperCase(c);
    Node oldNode = header;
    header = new Node();
    header.setData(c);
    header.setNext(oldNode);
  }

  public char pop() {
    Node temp = new Node();
    char data;
    if (isEmpty()) {
        System.out.printf("Stack Underflow (pop)\n");
        System.exit(0);
    }
    temp = header;
    data = temp.getData();
    header = header.getNext();
    return data;
  }

}

public class Queue {

  public void enqueue(char c) {
    c = Character.toUpperCase(c);
    Node n = last;
    last = new Node();
    last.setData(c);
    last.setNext(null);
    if (isEmpty()) {
        first = last;
    } else n.setNext(last);     
  }

  public char dequeue() {
    char data;
    data = first.getData();
    first = first.getNext();
    return data;
  }

}

public String cleanString(String s) {
    return s.replaceAll("[^A-Za-z0-9]", "");
}

Basically, when running my code through the debugger in Eclipse, my pop and dequeue methods appear to only select certain alphanumerics. I am using replaceAll("[^A-Za-z0-9]", "") to “clean” the user’s string of any nonalphanumeric chars (!, ?, &, etc.). When I say it only selects certain chars, there doesn’t seem to be any pattern that I can discern. Any ideas?

Answer 1

Your general algorithm works properly, assuming your queue and stack are correct (i tried this using the Deque implementations found in the jdk). Since your assignment involves the datastructures, i’ve pretty much just took your main logic and replaced the datastructures with ArrayDequeue, so I don’t feel like i’m answering this for you.

    String word = "ooffoo";

    word = word.replaceAll("[^A-Za-z0-9]", "");

    Deque<Character> stack = new ArrayDeque<Character>(word.length());
    Deque<Character> queue = new ArrayDeque<Character>(word.length());

    for (char c : word.toCharArray()) {
        stack.push(c);
        queue.add(c);
    }

    boolean pal = true;

    while (! stack.isEmpty() && pal == true) {
        if (! stack.pop().equals(queue.remove())) {
            pal = false;
        }
    }

    System.out.println(pal);