原题链接: https://oj.leetcode.com/problems/add-two-numbers/

public ListNode addTwoNumbers(ListNode left, ListNode right) {
    if (left == null && right == null) {
        return null;
    }

    ListNode result = new ListNode(0);
    ListNode cursor = result;
    int carry = 0;

    while (left != null || right != null) {
        int val = 0;

        if (left != null) {
            val += left.val;
            left = left.next;
        }

        if (right != null) {
            val += right.val;
            right = right.next;
        }

        val += carry;
        int digit = val % 10;
        carry = val / 10;

        cursor.val = digit;

        if (left != null || right != null) {
            cursor.next = new ListNode(0);
            cursor = cursor.next;               
        }
    }

    if (carry > 0) {
        cursor.next = new ListNode(1);
    }

    return result;
}