2. Add Two Numbers

对两个链表从起始节点进行相加，返回起始节点

• Runtime: 140 ms, faster than 52.29%
• Memory Usage: 44.5 MB, less than 22.68%

/**
 * 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 start =  new ListNode(0)
    let res = start
    let index = 0
    let carry = 0
    while(l1 || l2) {
        let val1 = l1 ? l1.val : 0
        let val2 = l2 ? l2.val : 0
        let sum = val1 + val2 + carry
        res.next = new ListNode(sum % 10)
        carry = parseInt(sum / 10 )
        if (l1) l1 = l1.next
        if (l2) l2 = l2.next
        res = res.next
    }
    if(carry !== 0) res.next = new ListNode(carry)
    return start.next
};