"Add Two Numbers" JavaScript Leetcode error

Question

I am getting errors in my leetcode and I am not sure why:

var addTwoNumbers = function(l1, l2) {
let newL1 = []
let newL2 = []
let answer = []

for(let i = 0; i < l1.length; i++) {
    newL1[i] = l1[l1.length - 1 - i]
}

for(let i = 0; i < l2.length; i++) {
    newL2[i] = l2[l2.length - 1 - i]
}

let num = parseInt(newL1.toString().replace(/,/g, '')) + parseInt(newL2.toString().replace(/,/g, ''))

let rawAnswer = (num.toString().split(""))

for(let i=0; i < rawAnswer.length; i++) {
    answer[i] = parseInt(rawAnswer[i])
}

return answer

}

Error:

Line 45 in solution.js
             throw new TypeError(__serialize__(ret) + " is not valid value for the expected return type ListNode");
             ^
TypeError: null is not valid value for the expected return type ListNode
    Line 45: Char 20 in solution.js (Object.<anonymous>)
    Line 16: Char 8 in runner.js (Object.runner)
    Line 29: Char 26 in solution.js (Object.<anonymous>)
    Line 1251: Char 30 in loader.js (Module._compile)
    Line 1272: Char 10 in loader.js (Object.Module._extensions..js)
    Line 1100: Char 32 in loader.js (Module.load)
    Line 962: Char 14 in loader.js (Function.Module._load)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:72:12)
    Line 17: Char 47 in run_main_module.js

Challenge Description:

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:

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

I am not sure why I am getting this error, but I know I am doing something that leetcode doesn't like.

Thanks

Answer 1

The description reads: return the sum as a linked list

You're doing two parseInt and returning the sum (that is a number) but it should return a linked list instead, defined by the head of the list (the first ListNode object.

Answer 2

var addTwoNumbers = function(l1, l2) {
        let k=new ListNode(0,null);
        let k1= k;
        let x,y,c=0;
        while(l1!=null || l2!=null){
            x = l1!=null?l1.val:0;
            y = l2!=null?l2.val:0;
            c=x+y+c;
            k1.next =new ListNode(c%10,null);
            k1 = k1.next ;
            c=parseInt(c/10);
            if(l1!=null) l1=l1.next ;
            if(l2!=null) l2=l2.next ;
        }
        
        if(c>0) {
            k1.next=new ListNode(c,null);
            k1=k1.next ;
        }
      
        return k.next;

};

ListNode this function .

1. value.next - next item
2. value.val - the value of that element