/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        int cur = 0;
        int res = 0;
        ListNode* H = new ListNode();
        ListNode* L = H;
        int temp = 0;
        while(l1||l2){
            // 避免两个链表不一样长的情况
            if(!l1)
                temp = l2->val;
            else if(!l2)
                temp = l1->val;
            else
                temp = l1->val+l2->val;
            cur = (temp + res)%10; // 求当前位
            res = (temp+res)/10;  // 求进位
            ListNode* p = new ListNode(cur);
            H->next = p;
            H = H->next;
            if(l1) l1 = l1->next;
            if(l2) l2 = l2->next;
        }
        if(res>0) H->next = new ListNode(res);  // 如果最后一位进位大于0，则添加
        return L->next;
    }
};