2_add_two_number.cpp

class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        ListNode *dummy = new ListNode();
        ListNode *temp = dummy;
        int carry = 0;
        while (l1 != NULL || l2 != NULL || carry) {
            int sum = 0;
            if (l1 != NULL) {
                sum += l1->val;
                l1 = l1->next;
            }
            if (l2 != NULL) {
                sum += l2->val;
                l2 = l2->next;
            }
            sum += carry;
            carry = sum / 10;
            ListNode *Node = new ListNode(sum % 10);
            temp->next = Node;
            temp = temp->next;
        }
        return dummy->next;
    }
};