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    1. public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    2.         ListNode head = new ListNode(-1);
    3.         ListNode pre = head;
    4.         int carry = 0;
    5.         while (l1 != null && l2 != null) {
    6.             ListNode node = new ListNode();
    7.             int sum = l1.val + l2.val + carry;
    8.             carry = sum / 10;
    9.             node.val = sum % 10;
    10.             head.next = node;
    11.             head = head.next;
    12.             l1 = l1.next;
    13.             l2 = l2.next;
    14.         }
    15.         while (l1 != null) {
    16.             ListNode node = new ListNode();
    17.             int sum = l1.val + carry;
    18.             carry = sum / 10;
    19.             node.val = sum % 10;
    20.             head.next = node;
    21.             head = head.next;
    22.             l1 = l1.next;
    23.         }
    24.         while (l2 != null) {
    25.             ListNode node = new ListNode();
    26.             int sum = l2.val + carry;
    27.             carry = sum / 10;
    28.             node.val = sum % 10;
    29.             head.next = node;
    30.             head = head.next;
    31.             l2 = l2.next;
    32.         }
    33.         if (carry != 0) {
    34.             head.next = new ListNode(carry);
    35.         }
    36.         return pre.next;
    37.     }