Stacks and Queues




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In this assignment, you will be implementing Stacks and Queues. You will then use stacks and
queues to test if words are palindromes, and use stacks to decompose phrases into words (to
find new palindromes).
● Implement MyStack (use the included MyLinkedList to store the stack elements)
○ Implements StackInterface
○ Throws a StackException if peek or pop are called on an empty stack
● Implement MyQueue (use MyLinkedList to store the stack elements)
○ Implements QueueInterface
○ Throws a QueueException if peek or dequeue are called on an empty queue
● Test your MyStack and MyQueue thoroughly
● In
○ Implement stackToReverseString() using MyStack
○ Implement reverseStringAndRemoveNonAlpha() using MyStack
○ Implement isPalindrome(), which returns true if a word or phrase is a
palindrome, using MyStack and MyQueue
○ CHALLENGE: Implement explorePalindrome() which lists all possible
backwards decompositions of a phrase (e.g. “evil guns” => snug live“), a common
trick to make new palindromes (uses MyStack)
● Style requirements (NEW!)
○ Indent your code properly
■ There are several mainstream “styles” of indentation, pick one and be
consistent. EG:
○ Name your variables with helpful and descriptive names
■ Whats a good variable name? Here is a guide
○ Add comments before every function, and in your code
■ Comments should say what you are trying to do and why
■ Consult this guide to commenting
Point breakdown
Stacks – 20 points
Queues – 20 points
Style – 15 points
Implementing the functions:
String stackToReverseString(MyStack) – 10 points
String reverseStringAndRemoveNonAlpha(String) – 5 points
Boolean isPalindrome(String) – 10 points
void explorePalindrome() (and its helper) – 20 points
Implementing MyStack and MyQueue
In this assignment, we will be making heavy use of the classes MyStack and MyQueue. You
have already implemented in a previous assignment. You can use your
code, or the sample provided.
Implement and using the provided interfaces and exceptions.
Make sure you have implemented a public String toString() method on MyStack and
MyQueue that prints out the contents of the stack from the top down and prints out the queue
from the front to back. So, for example, after the following code:
MyStack stack = new MyStack();
MyQueue queue = new MyQueue();
System.out.println(“Stack = ” + stack);
System.out.println(“Queue = ” + queue);
Then the output would be:
Stack = (world, big, hello)
Queue = (hello, big, world)
Test your code thoroughly!! We have provided as an example
of some ways to test your code, but you will want to edit it to try out many possible situations.
Make sure your code behaves exactly as you expect it to, before starting the second half of the
Is this a palindrome?
A palindrome is a word or phrase that reads the same backwards and forwards, if you ignore
punctuation and spaces. For example:
● A dog! A panic in a pagoda!
● Cigar? Toss it in a can. It is so tragic.
● Yo, bottoms up! (U.S. motto, boy.)
● Stressed was I ere I saw desserts.
In this part of the assignment, you will be writing several functions in to test
and create palindromes using your stack and queue implementations.
Example Input & Output takes as input: a mode, and some words. The mode is either “test” or
“expand”, so the function call will be:
Test for palindromes
Are these phrases palindromes?
javac && java Palindrome test “oboe” “ABBA” “I’m alas, a
salami” “evil liver”
‘oboe’: false
‘ABBA’: true
‘I’m alas, a salami’: true
‘evil liver’: false
Expand palindromes
Which words could be added to make this a palindrome?
javac && java Palindrome expand “an era live” “mug god”
an era live: evil a ren a
an era live: evil aren a
an era live: evil arena
mug god: dog gum
Functions to implement:
String stackToReverseString(MyStack): your
toString function in your stack class prints out the
stack in the order that things would be popped from
it. What if we want to turn it into a string in the
opposite order (the order that things were pushed
to it)?
In, we do not have access to the
internal representation of the stack’s list. So
instead, we have to pop everything off the stack,
read it in order and push it pack onto the stack in
the original order so that the stack is unchanged
● Create an empty string
● Create a new temporary stack
● Pop everything from the original stack onto
the new stack
● Pop everything from the new stack back
onto the original stack and add it to the
String reverseStringAndRemoveNonAlpha (String)
Use a stack to reverse a string. Similar to before. Also, we want to not only
● Create a new stack.
● Iterate through the string, and push each character from the string onto the stack, but
only if they are alphabetic (ignore spaces and punctuation)
○ Character.isAlphabetic will be a useful function here!
● Pop everything from the stack to reconstruct the string in reverse.
● Note: your stack can only contain Objects. Java’s char datatype isn’t an object though!
This means that you will have to wrap it (and later cast it) as a Character type. Look up
the Character class in Java’s documentation, or find out more about wrapper classes
Boolean isPalindrome(String)
Implement this function using both a stack and a queue.
To test if a string is a palindrome:
● Convert the string to lowercase (we don’t care about uppercase vs lowercase characters
being different)
● Create a new stack and queue.
● Enqueue and push each character (if it is alphabetic, we don’t want to look at white
space or punctuation)
● Pop each character from the stack and dequeue each character from the queue until
one is empty
● Notice how in our above functions, pushing and then popping from a stack reversed the
order. How will you use this to test whether this string is a palindrome?
void explorePalindrome(String)
This function lists all possible endings that would make this string a palindrome, e.g.:
javac && java Palindrome expand “an era live” “mug god”
an era live: evil a ren a
an era live: evil aren a
an era live: evil arena
First, convert the string to lowercase and use your reverseStringAndRemoveNonAlpha
function to reverse it and remove non-alphabetical characters.
Now, most of the work will be done by a recursive helper function to decompose this new string
(“evilarena”) into words. Takes the original string, the reversed string, an index, and the current
stack of words we are building up
public static void decomposeText(String originalText, String
textToDecompose, int index, MyStack decomposition)
We have provided a function String[] getWords(String text, int index) that uses a
dictionary to find which words could be created from a string at a given index. For example
getWords(“isawere”, 0) could find the words “i” and “is”, getWords(“isawere”, 2) could
find the words (“a”, “aw” and “awe”).
A recursion step:
● If the index is at the end of the word, we are finished, print out the words (using
reverse print) and the original text.
● Else: Find the potential words at that index
● For each word:
● Push it to the stack
○ Recurse at the next index (not *just* i++)
○ If it was part of a correct solution, it will print itself out in a subsequent
recursion step (if it reaches a conclusion)
● Pop it from the stack
● Confused? See below for a visual explanation of this approach.
○ As usual, println statements may help you understand your code’s operation if
you get lost. Consider outputting the list of words from getWords, or printing out
the decomposition stack each time you push or pop from it. Just remove your
debug statements before turning in your code.
Turning the code in
● Create a directory with the following name: _assignment3 where
you replace with your actual student ID. For example, if your
student ID is 1234567, then the directory name is 1234567_assignment3
● Put a copy of your edited files in the directory (,, and
● Compress the folder using zip. Zip is a compression utility available on mac, linux
and windows that can compress a directory into a single file. This should result in
a file named (with replaced with
your real ID of course).
● Double-check that your code compiles and that your files can unzip properly.
You are responsible for turning in working code.
● Upload the zip file through the page for Assignment 3 in canvas
Visual example of palindrome search with stacks