linkedList.cpp

/**
 * 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) {
        int sum=0;
        ListNode *l3 = NULL;
        ListNode **node = &l3;
        
        while (l1 != NULL || l2 != NULL || sum > 0){
            
            if (l1 != NULL){
                sum += l1->val;
                l1 = l1->next;
            }
            
            if (l2 != NULL){
                sum += l2->val;
                l2 = l2->next;
            }
                    
            *node = new ListNode(sum % 10);
            node = &((*node)->next);
            sum /= 10;
        }
        
        return l3;
    }
};