Add Two Numbers #2 From Leetcode = You are given two non-empty linked lists representing two non-negative integers. The most significant digit comes first and each of their nodes contain a single digit. Add the two numbers and return it as a linked list. You may assume the two numbers do not contain any leading zero, except the number 0 itself.

sum-linked-lists.js

/**
 * 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 revL1 = reverse(l1), revL2 = reverse(l2), carry = 0, l3
    
    while(revL1 || revL2) {
        let sum = (revL1 !== null ? revL1.val : 0) + (revL2 !== null ? revL2.val : 0) + carry
        
        carry = sum >= 10 ? 1 : 0
        let lastDigit = sum % 10
        let newNode = new ListNode(lastDigit)

        if (l3 == null) {
            l3 = newNode
        } else {
            newNode.next = l3
            l3 = newNode
        }
        
        if (revL1 != null) revL1 = revL1.next
        if (revL2 != null) revL2 = revL2.next
    }
    
    if (carry > 0) {
        let newNode = new ListNode(carry)
        newNode.next = l3
        l3 = newNode 
    }
    
    return l3
};

var reverse = function(head) {
    let prev = null
    while(head){
        let next = head.next
        head.next = prev
        prev = head
        head = next
    }       
    return prev;
}

Input: (7 -> 2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 8 -> 0 -> 7