2 两数相加

https://leetcode.cn/leetbook/read/top-interview-questions-medium/xvw73v/

cost: 27m58s

/**
 * 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 head = l1;
        int carry = 0;
        while (l1 != nullptr && l2 != nullptr) {
            int val = l1->val + l2->val + carry;
            carry = val / 10;
            l1->val = val % 10;
            if (l1->next == nullptr || l2->next == nullptr) {
                break;
            }
            l1 = l1->next;
            l2 = l2->next;
        }
        if (l1->next == nullptr) {
            l1->next = l2->next;
        }
        if (l1->next == nullptr) {
            if (carry != 0) {
                l1->next = new ListNode(carry);
                return head;
            }
        }
        l1 = l1->next;
        while (l1 != nullptr) {
            int val = l1->val + carry;
            carry = val / 10;
            l1->val = val % 10;
            if (l1->next == nullptr) {
                break;
            }
            l1 = l1->next;
        }
        if (carry != 0) {
            l1->next = new ListNode(carry);
        }
        return head;
    }
};