SOURCE

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 addTwoNumbers(l1: ListNode | null, l2: ListNode | null): ListNode | null {
    let b = 0;
    let start = null;
    let temp = null;
    while( l1 || l2){
        let sum = 0;
        sum = (l1?l1.val:0) + (l2?l2.val:0) + b
        b = Math.floor(sum/10)
        // console.log(sum%10)
        if(!start){
            start = temp = new ListNode(sum%10)
        }else{
            temp.next = new ListNode(sum%10)
            temp = temp.next
        }
        l1 = l1?l1.next:null;
        l2 = l2?l2.next:null;
        if(b){
            temp.next = new ListNode(b)
        }
    }
    return start
};

// 342
const one = new ListNode(5);
// one.next = new ListNode(4);
// one.next.next = new ListNode(3);
// 465
const two = new ListNode(7);
// two.next = new ListNode(6);
// two.next.next = new ListNode(4);

console.log(addTwoNumbers(one,two));