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) {}
 * };
 */

 // 模拟
class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        ListNode* head = nullptr;
        ListNode* cur = head;
        int carry = 0;
        while(l1 || l2) {
            int num1 = l1? l1->val: 0;
            int num2 = l2? l2->val: 0;
            int sum = num1 + num2 + carry;

            if (head == nullptr) {
                head = new ListNode(sum % 10);
                cur = head;
            } else {
                cur->next = new ListNode(sum % 10);
                cur = cur->next;
            }

            carry = sum / 10;
            if (l1) l1 = l1->next;
            if (l2) l2 = l2->next;  
        }

        // 最后一位进位
        if (carry > 0) cur->next = new ListNode(carry);

        return head;
    }
};