Question

(Palindrome Tester) Write a program that uses a stack to determine whether a string is a palindrome (i.e., the string is spelled identically backward and forward). The program should ignore spaces and punctuation.

Short answer

A program to check for palindromes should normalize the input string by removing non-letter characters and changing to a common case, use a stack to compare characters, and return true if all characters match, otherwise false.

Step-by-step solution

Define a Palindrome

Understand that a palindrome is a word, phrase, number, or other sequences of characters that reads the same forward and backward (ignoring spaces, punctuation, and capitalization).

Normalize the Input String

Remove all non-letter characters from the string and convert all letters to the same case (upper or lower) to ensure consistency when comparing characters.

Initialize a Stack

Create an empty stack data structure that will be used to hold characters from the string.

Populate the Stack

Iterate over the normalized string and push each character onto the stack.

Compare Characters

While iterating over the normalized string again, pop characters from the stack and compare them to the current character in the string. If there is a mismatch, the string is not a palindrome.

Return the Result

If the stack is exhausted and all characters have matched, conclude that the string is a palindrome. Otherwise, conclude that it is not.

Key concepts

Stack Data Structure

Imagine a stack of books on your desk. When you add a new book, you place it on top of the last one, and when you take one out, you remove it from the top of the pile. This 'last in, first out' (LIFO) principle is at the heart of a stack data structure. In a palindrome tester algorithm, a stack plays a vital role by helping to reverse the characters of the string.

You start by pushing characters onto the stack one by one. Think of each character as a new book added to your pile. Later, when you're checking if the string is a palindrome, you'll be removing the characters (books) from the top of the stack, which effectively reverses the order of the characters.

This reversal is critical because to determine if a string is a palindrome, you must compare the first character with the last, the second with the second last, and so on. The stack, with its simple yet powerful structure, makes this process intuitive and efficient.

String Normalization

When you're testing for palindromes, details like capitalization, spaces, and punctuation can throw a wrench into your comparison process – imagine trying to read a book where the case of letters changes on every page, or where punctuation marks are scattered randomly. That's where string normalization comes in – think of it as cleaning up the text so that you're left with a consistent, easy-to-read format.

Normalizing a string usually involves two steps: removing any characters that are not part of the comparison (like spaces and punctuation), and making sure that the remaining characters are all in the same case, either upper or lower. This ensures that when you compare the characters to check for a palindrome, you're comparing like with like – similar to ensuring that all the books in your stack are facing the same way and none are upside-down or sideways.

Character Comparison

Character comparison is the detective work in our palindrome investigation. It involves looking at two characters and deciding if they are identical. But this isn't just a simple glance; in a palindrome tester, you must be meticulous, comparing characters across the length of the string.

After normalizing the string and stacking the characters, you then go through the string and start the comparisons. You remove the top character from the stack (the end of the string, due to reversal) and compare it with the current character from the front of the string. If at any point the characters don't match, you know the string isn't a palindrome – akin to discovering that a puzzle piece doesn't fit and realizing it's from a different jigsaw puzzle.

Completing this process across the entire string ensures that you examine the string from both ends, meeting in the middle. If all the characters match, you've successfully discovered a palindrome!