Determining Palindromes (Leet code problem solving Part 3 )

In this series I'll be solving Leetcode problems for SDETs one by one.

Problem

125. Valid Palindrome

A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers.

Given a string s, return true if it is a palindrome, or false otherwise.

Example 1:

Input: s = "A man, a plan, a canal: Panama" Output: true Explanation: "amanaplanacanalpanama" is a palindrome.

Example 2:

Input: s = "race a car" Output: false Explanation: "raceacar" is not a palindrome.

Example 3:

Input: s = " " Output: true Explanation: s is an empty string "" after removing non-alphanumeric characters. Since an empty string reads the same forward and backward, it is a palindrome.

Constraints:

• 1 <= s.length <= 2 * 105
• s consists only of printable ASCII characters.

Code Walkthrough:

• The function begins by checking if the string is empty, in which case it is considered a palindrome.
• Two pointers, start and last, are initialized to the beginning and end of the string, respectively.
• The main loop continues until the pointers meet in the middle of the string.
• At each step, the characters at the current positions are compared after handling non-alphanumeric characters appropriately.
• If a mismatch is found, the function returns false.
• If the loop completes, the string is deemed a palindrome.

class Solution {
    public boolean isPalindrome(String s) {
        if (s.isEmpty()) {
        	return true;
        }
        int start = 0;
        int last = s.length() - 1;
        while(start <= last) {
        	char currFirst = s.charAt(start);
        	char currLast = s.charAt(last);
        	if (!Character.isLetterOrDigit(currFirst )) {
        		start++;
        	} else if(!Character.isLetterOrDigit(currLast)) {
        		last--;
        	} else {
        		if (Character.toLowerCase(currFirst) != Character.toLowerCase(currLast)) {
        			return false;
        		}
        		start++;
        		last--;
        	}
        }
        return true;
    }
}

Using string builder

    public static boolean isPalindrome(String s) {
        s = s.replaceAll("[^a-zA-Z0-9]", "").toLowerCase();
        return s.contentEquals(new StringBuilder(s).reverse());
    }

JS code :

function isPalindrome(s) {
    if (s.length === 0) {
        return true;
    }

    let start = 0;
    let last = s.length - 1;

    while (start <= last) {
        const currFirst = s.charAt(start);
        const currLast = s.charAt(last);

        if (!isLetterOrDigit(currFirst)) {
            start++;
        } else if (!isLetterOrDigit(currLast)) {
            last--;
        } else {
            if (currFirst.toLowerCase() !== currLast.toLowerCase()) {
                return false;
            }
            start++;
            last--;
        }
    }

    return true;
}

function isLetterOrDigit(char) {
    return /^[0-9a-zA-Z]$/.test(char);
}

// Test cases
const s1 = "A man, a plan, a canal: Panama";
console.log(isPalindrome(s1)); // Output: true

const s2 = "race a car";
console.log(isPalindrome(s2)); // Output: false

Code files : https://github.com/Bosco98/Practice/blob/main/src/palindromeAlphaNumeric.java

Entire repo : https://github.com/Bosco98/Practice/

JS CODE : https://github.com/Bosco98/Practice/tree/main/js%20code