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class Solution {
   
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        //虚拟头节点(哑结点)
        ListNode dummy = new ListNode(0);
        ListNode temp = dummy;
        //进位
        int carry = 0;
        while(l1!=null || l2!=null || carry>0){
            int i = l1==null? 0 : l1.val;
            int j = l2==null? 0 : l2.val;
            //相加之和(包含进位的)
            int sum = i + j +carry;
            if(sum>=10){
                //尾插法连接
                temp.next = new ListNode(sum%10);
            }else{
                temp.next = new ListNode(sum);
            }
            //更新进位的值
            carry = sum / 10;

            temp = temp.next;
            if(l1!=null){
                l1 = l1.next;
            }
            if(l2!=null){
                l2 = l2.next;
            }
        }
        return dummy.next;
    }
}