/**
 * 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)
    {
        ListNode *head = nullptr; //定义头结点为空
        ListNode *tail = nullptr;
        int carry = 0, sum = 0;
        while (l1 || l2){
            int n1 = l1 ? l1->val : 0;  //l1有value, 取出给n1
            int n2 = l2 ? l2->val : 0;
            sum = n1 + n2 + carry;
            if (!head){ // 头结点为空，新链表没有节点
                head = tail = new ListNode(sum % 10); //插入新节点
            } else { //有头结点，后续节点插入
                tail->next = new ListNode(sum % 10); //尾节点后插入
                tail = tail->next;
            }
            carry = sum / 10;  //进位值
            if (l1){ // l1还有节点继续访问
                l1 = l1->next;
            }
            if (l2){
                l2 = l2->next;
            }
        }

        if (carry > 0) {// 最后一个进位值放到链表最 
            tail->next = new ListNode(carry);
            tail = tail->next;
        }
        
        return head; // 返回头结点即可

    }
};