002. Add Two Numbers

//C++ solution:

/**
 * 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==NULL) {return l2;}
        if (l2==NULL) {return l1;}
        
        ListNode* res = NULL;
        ListNode* result = NULL;
        
        int value = 0;
        int carry = 0;
        
        while(l1 != NULL || l2 != NULL) {
            value = carry;
            if(l1 != NULL) {
                value = value + l1->val;
            }
            if(l2 != NULL){
                value = value + l2->val;
            }
            carry = value / 10;
            value = value - carry * 10;
            
            if(res == NULL){
                res = new ListNode(value);
                result = res;
            }
            else{
                res->next = new ListNode(value);
                res = res->next;
            }
            
            if(l1 != NULL){
                l1 = l1->next;
            }
            if(l2 != NULL){
                l2 = l2->next;
            }
        }
        
        if(carry != 0){
            res->next = new ListNode(carry);
        }
        
        return result;
    }
};