Add Two Numbers

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        //ListNode *res = l1;
        int c = 0;
        ListNode *back, *res=l1;
        
        while (l1!=NULL && l2!=NULL)
        {
            int tmp = l1->val+l2->val+c;
            l1->val = tmp%10;
            c = tmp/10;
            l2 = l2->next;
            back = l1;
            l1 = l1->next;
        }
        
        ListNode *contin = (l2==NULL)?l1:l2;
        back->next = contin;
        
        while (contin)
        {
            int tmp = contin->val+c;
            contin->val = tmp%10;
            c = tmp/10;
            back = contin;
            contin = contin->next;            
        }
        
        if(c)
        {
            ListNode *high = new ListNode(c);
            back->next = high;
        }

        return res;
    }
};