2. 两数相加

/**
 * 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) {
        auto sum=new ListNode(-1),cur=sum;
        int t=0;
        while(l1||l2||t)
        {
            if(l1) t+=l1->val,l1=l1->next;
            if(l2) t+=l2->val,l2=l2->next;
            cur=cur->next=new ListNode(t%10);
            t/=10;
        }
        return sum->next;
    }
};