You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.
Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
给你两个链表，这两个链表代表两个数字，从头到尾为低位到高位。
这两个数做相加，然后以同样形式的链表返回结果
同时遍历两个链表，同时逐位建立结果链表

var addTwoNumbers = function(l1, l2) {
    var up = 0;
    var head = null;
    var last = null;
    var head1 = l1;
    var head2 = l2;
    while (head1||head2) {
        let a = head1 ? head1.val : 0;
        let b = head2 ? head2.val : 0;
        let res = up + a + b;
        up = parseInt(res/10);
        res = res%10;
        res = new ListNode(res);
        if (head) {
            last.next = res;
            last = res;
        } else {
            head = res;
            last = res;
        }
        if (head1) head1 = head1.next;
        if (head2) head2 = head2.next;
    }
    if (up!==0) {
        let res = new ListNode(up);
        last.next = res;
        last = res;
    }
    return head;
};