addTwoNumber.ts

class ListNode {
	val: number;
	next: ListNode | null;
	constructor(val?: number, next?: ListNode | null) {
		this.val = val === undefined ? 0 : val;
		this.next = next === undefined ? null : next;
	}
}

function getNumber(linkedList: ListNode | null): string {
	let result = linkedList.val.toString();
	let nextNodeValue = linkedList.next;

	while (nextNodeValue !== null) {
		result = nextNodeValue.val.toString() + result;
		nextNodeValue = nextNodeValue.next;
	}

	return result;
}

function plusStringNumber(firstNumber: string, secondNumber: string): string {
	let tempPlusValue = 0;
	let arrayLength = secondNumber.length;
	let resultPlus = '';

	// Add temp zero number at the beginning of the number has less digits
	let tempZero = Array.from(
		{ length: Math.abs(firstNumber.length - secondNumber.length) },
		() => '0'
	).join('');

	if (firstNumber.length < secondNumber.length) {
		firstNumber = tempZero + firstNumber;
	} else {
		secondNumber = tempZero + secondNumber;
		arrayLength = firstNumber.length;
	}

	// Plus from last index digit to first index digit
	for (let index = arrayLength - 1; index >= 0; index--) {
		let plus = Number(firstNumber[index]) + Number(secondNumber[index]) + tempPlusValue;
		tempPlusValue = 0;

		if (plus >= 10) {
			tempPlusValue = Math.floor(plus / 10);
			plus = plus % 10;
		}

		resultPlus = plus.toString() + resultPlus;
	}

	// Add 1 at the beginning of result string
	if (tempPlusValue > 0) {
		resultPlus = '1' + resultPlus;
	}

	return resultPlus;
}

function addTwoNumbers(l1: ListNode | null, l2: ListNode | null): ListNode | null {
	let firstNumber = getNumber(l1);
	let secondNumber = getNumber(l2);
	const plus = plusStringNumber(firstNumber, secondNumber);
	const resultLinkNode = new ListNode();
	let node = resultLinkNode;

	const digits = plus.split('');
	console.log(digits);

	for (let index = digits.length - 1; index >= 0; index--) {
		node.val = Number(digits[index]);
		node.next = index === 0 ? null : new ListNode();
		node = node.next;
	}

	return resultLinkNode;
}

const firstLinkedList = new ListNode(9, new ListNode(9, new ListNode(9, new ListNode(9))));
const secondLinkedList = new ListNode(9, new ListNode(9, new ListNode(9)));

console.log(addTwoNumbers(firstLinkedList, secondLinkedList));