2. Add Two Numbers

You are given two non-empty linked lists representing two non-negative integers. 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.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

链表存的是数字的反序，实际是342+465=807

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
  ListNode dummy = new ListNode(-1);
  ListNode cur = dummy;

  int carry=0;
  while (l1 != null || l2 != null) {
    int sum = 0;
    if (l1 != null) {
      sum += l1.val;
      l1 = l1.next;
    }
    if (l2 != null) {
      sum += l2.val;
      l2 = l2.next;
    }
    sum += carry;
    ListNode node = new ListNode(sum%10);
    carry = sum / 10;
    cur.next = node;
    cur = cur.next;
  }
  if (carry != 0) {
    ListNode node = new ListNode(1);
    cur.next = node;
  }
  return dummy.next;
}