LeetCode : Add Two Numbers

add-two-numbers.js

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var addTwoNumbers = function(l1, l2) {

    let firstNode = null;
    let prevNode = null;
    let plus = 0;
    while (l1 || l2 || plus > 0) {
        if(!l1) {
            l1 = new ListNode(0);
        }
        if (!l2) {
            l2 = new ListNode(0);
        }
        let nowVal = (l1.val + l2.val + plus);
        if (nowVal - 10 >= 0) {
            nowVal -= 10;
            plus = 1;
        } else {
            plus = 0;
        }
        const newNode = new ListNode(nowVal);
        if (!firstNode) {
            firstNode = newNode;
            prevNode = firstNode;
        } else {
            prevNode.next = newNode;
            prevNode = newNode;   
        }
        l1 = l1.next;
        l2 = l2.next;
    }
    
    return firstNode;
};