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class Solution {//只能从低位往高位加
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        auto dummy = new ListNode(-1), cur = dummy;
        int t = 0;
        while(l1 || l2 || t){
            if(l1){
                t += l1->val;
                l1 = l1->next;
            } 
            if(l2){
                t += l2->val;
                l2 = l2->next;
            }   
            cur->next = new ListNode(t % 10);
            cur = cur->next;
            t = t / 10;
        }
        return dummy->next;
    }
};