Palindrome Number

Introduction

Hello and welcome! I’m Ming Lee, and today we’re going to cover How to solve LeetCode 9 - Palindrome Number - in JS/TS ⚡

Same as always we’re gonna solve this, and we’re gonna do it without the big fancy words because this is for regular folx! Keeping it nice and casual round these parts! Movin’ on…

Problem

LeetCode 9 - Palindrome Number

Given an integer x, return true if x is a palindrome, and false otherwise.

Important Keywords

Palindrome : a word (or number) that is the same both forwards and backwards

Explanation

Nice! So this is a test of “do you know your JavaScript methods”? Now array’s have a really neat .reverse() method, and we’d love to use that, but sadly numbers don’t have access to that. Strings can split() into an array though, and we can always .toString() numbers into strings, so that’s our plan of action on how to gain access! Once we can reverse the number we want to turn it back into a number from its reversed array counterpart and compare that to the original. So the steps should look something like this:

Do a .toString() on a number to get the string version
Do a .split(’’) on that string to get an array
Perform the .reverse()! ⚡
Do a .join(’’) to return to a string
Cast the string back into a number with either + or Number()

As always, this will be done in TypeScript, but all my JS folx don’t be discouraged, this should be totally readable! Try and ignore all the colons and the types that follow them. The code should look super familiar if you can get past that!

Solution

function isPalindrome(num: number): boolean {
    return num === +num.toString().split('').reverse().join('');
}

Refactor Explaination

So the above code is nice, short, and sweet. The problem is that we didn’t really get a good chance to flex those big brains of ours! Fortunately LeetCode to the rescue by asking us a follow up! Can we solve it without converting the integer to a string? You betcha! The downside is that its gonna take a weeee bit of work.

Lets consider this. We’ll make a new variable, reversed, and we’ll try and pop the ‘ones’ digit value off of 123. So to remove that ones digit, we need to use a modulo operation. 123 % 10 is 3, which is exactly what we need! So we’ll add that 3 to reversed. We do that with something like reversed = reversed * 10 + (the remainder). This shifts all the other digits to the right to allow for the new incoming digit. Hopefully the table below with clarify the end results further. Now to remove the ones digit from 123 itself we need to do something like num = Math.floor(num / 10). In this example, 123 becomes 12.3, but the floor of that is 12. We continue this until num becomes 0.

Step	reversed	num
0	0	123
1	3	12
2	32	1
3	321	0

Now seeing that this just reversed the num, but that num is now 0, we know that we need another variable to hold the initial value of num to compare. So we’ll take that into consideration as well. This seems like enough chit-chat, lets get to slappin keyboards!

P.S. - One important note here! Negatives don’t do so well in our reverser, but fortunately, due to the - sign, negatives can never be palindromes! So uh… if the num is less than zero, we always gotta return false.

Refactor Solution

function isPalindrome(num: number): boolean {
    if(num < 0) return false;

    let original = num,
        reversed = 0;

    while(num){
        reversed = reversed * 10 + num % 10;
        num = Math.floor(num / 10);
    }

    return reversed === original;
}

Closing

And there’s some working code for checking if a number is a palindrome, just without turning it into a string first! This is one of those kinds of follow up questions that kind of changes a lot of things. It ends up making the problem a completely different challenge, and turned this from a simple “do ya know how to use methods” to a kind of “can you manipulate numbers at different places, aka ones,tens,hundreds”. Its ok if this one feels a little funky, and its also totally ok if it feels a little mathy. The more you see this and the more you practice with it your brain will make those connections repeatedly and before ya know it these kinds of operations will be just another tool in the tool belt. Next on our list of LeetCode problems we’ll be taking down a super duper fun one! The dreaded LeetCode 13 - Roman to Integer. See you there!