lc 两数相加

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        if(l1 == nullptr && l2 == nullptr) return nullptr;
        ListNode* head = new ListNode(-1);
        ListNode* cur = head;
        int carry = 0;
        while(l1 || l2) {
            int sum = 0;
            if(l1) {
                sum += l1->val;
                l1 = l1->next;
            }
            if(l2) {
                sum += l2->val;
                l2 = l2->next;
            }
            if(carry) {
                sum += 1;
            }
            cur->next = new ListNode(sum%10);
            carry = sum / 10;
            cur = cur->next;
        }
        if(carry) {
            cur->next = new ListNode(1);
        }
        return head->next;
    }
};