## Problem:

You are given two **non-empty** linked lists representing two non-negative integers. The digits are stored in **reverse order**, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

**Example 1:**

Input:l1 = [2,4,3], l2 = [5,6,4]Output:[7,0,8]Explanation:342 + 465 = 807.

**Example 2:**

Input:l1 = [0], l2 = [0]Output:[0]

**Example 3:**

Input:l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]Output:[8,9,9,9,0,0,0,1]

**Constraints:**

- The number of nodes in each linked list is in the range
`[1, 100]`

. `0 <= Node.val <= 9`

- It is guaranteed that the list represents a number that does not have leading zeros.

## Solution:

### Brute force solution for beginners with separate functions for everything in a meaningful manner.

```
/**
* Definition for singly-linked list.
* function ListNode(val, next) {
* this.val = (val===undefined ? 0 : val)
* this.next = (next===undefined ? null : next)
* }
*/
/**
* @param {ListNode} l1
* @param {ListNode} l2
* @return {ListNode}
*/
var addTwoNumbers = function(l1, l2) {
let firstnumber = "";
let secondnumber = "";
while(l1){
firstnumber = firstnumber+""+l1.val;
l1 = l1.next;
}
while(l2){
secondnumber = secondnumber+""+l2.val;
l2 = l2.next;
}
let sum = BigInt(reversestringandreturnint(secondnumber)) + BigInt(reversestringandreturnint(firstnumber));
return linkedListFromArray(stringtoarray(sum+""));
};
function reversestringandreturnint(str){
let rev = "";
if(str){
for(let i = str.length -1;i>=0;i-- ){
rev = rev+""+str[i];
}
}
return rev;
}
function stringtoarray(str){
//reverse and convert to array
let arr = new Array();
if(str){
for(let i = str.length -1;i >=0;i--){
arr.push(str[i]);
}
}
return arr;
}
function linkedListFromArray(a) {
for (let i=0; i<a.length; i++) {
a[i] = new ListNode(a[i]);
if (i!=0 && i<a.length) {
a[i-1].next = a[i];
} else {
a[i].next = null;
}
}
return a[0];
}
function toNum(n) {
var nStr = (n + "");
if(nStr.indexOf(".") > -1)
nStr = nStr.replace(".","").replace(/\d+$/, function(m){ return --m; });
return nStr.replace(/(\d+)e\+?(\d+)/, function(m, g1, g2){
return g1 + new Array(+g2).join("0") + "0";
})
}
```

### Optimized solution using recursion for advanced developers :

```
/**
* Definition for singly-linked list.
* function ListNode(val, next) {
* this.val = (val===undefined ? 0 : val)
* this.next = (next===undefined ? null : next)
* }
*/
/**
* @param {ListNode} l1
* @param {ListNode} l2
* @return {ListNode}
*/
var addTwoNumbers = function(l1, l2) {
const iter = (n1, n2, rest = 0) => {
if (!n1 && !n2 && !rest) return null;
const newVal = (n1?.val || 0) + (n2?.val || 0) + rest;
const nextNode = iter(n1?.next, n2?.next, Math.floor(newVal / 10));
return new ListNode(newVal % 10, nextNode);
}
return iter(l1, l2);
};
```

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