// 模拟数学计算即可，主要复用原链表空间，不要 new 空间，否则会超出内存限制。
class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        if(l1 == nullptr) return l2;
        if(l2 == nullptr) return l1;

        auto dummy = new ListNode(-1);
        auto cur = dummy;
        int num = 0;
        while(l1 && l2) {
            int n = (l1->val + l2->val + num) % 10;
            num = (l1->val + l2->val + num) / 10;

            l1->val = n;
            auto next = l1->next;
            cur->next = l1;

            l1 = next;
            l2 = l2->next;
            cur = cur->next;    
        }

        while(l1) {
            int n = (l1->val + num) % 10;
            num = (l1->val + num) / 10;

            l1->val = n;
            auto next = l1->next;
            cur->next = l1;

            l1 = next;
            cur = cur->next;   
        }

        while(l2) {
            int n = (l2->val + num) % 10;
            num = (l2->val + num) / 10;

            l2->val = n;
            auto next = l2->next;
            cur->next = l2;

            l2 = next;
            cur = cur->next;
        }

        if (num != 0) {
            auto ret = new ListNode(num);
            cur->next = ret;

        }

        return dummy->next;
    }
};