链表题

public ListNode (ListNode l1, ListNode l2) {
    ListNode c1 = l1;
    ListNode c2 = l2;
    ListNode sentinel = new ListNode(0);
    ListNode d = sentinel;
    int sum = 0;
    while (c1 != null || c2 != null) {
        sum /= 10;
        if (c1 != null) {
            sum += c1.val;
            c1 = c1.next;
        }
        if (c2 != null) {
            sum += c2.val;
            c2 = c2.next;
        }
        d.next = new ListNode(sum % 10);
        d = d.next;
    }
    if (sum / 10 == 1)
        d.next = new ListNode(1);
    return sentinel.next;
}