2. 两数相加

https://leetcode-cn.com/problems/add-two-numbers/

/**
 * 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* dummy_head = new ListNode();
        ListNode* head = dummy_head;
        int acc = 0;
        while (l1 != nullptr || l2 != nullptr) {
            int val1(0), val2(0);
            if (l1 != nullptr) {
                val1 = l1->val;
                l1 = l1->next;
            }
            if (l2 != nullptr) {
                val2 = l2->val;
                l2 = l2->next;
            }
            int val = acc + val1 + val2;
            acc = val >= 10;
            head->next = new ListNode(val % 10);
            head = head->next;
        }
        if (acc) {
            head->next = new ListNode(1);
        }
        return dummy_head->next;
    }
};