Lintcode: Add Two Numbers

C++

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    /**
     * @param l1: the first list
     * @param l2: the second list
     * @return: the sum list of l1 and l2 
     */
    ListNode *addLists(ListNode *l1, ListNode *l2) {
        // write your code here
        if (l1 == NULL) {
            return l2;
        }
        if (l2 == NULL) {
            return l1;
        }
        ListNode *ptr1 = l1, *ptr2 = l2;
        ListNode *result = new ListNode(-1);
        ListNode *pre = result;
        int carry = 0, val = 0;
        while (ptr1 != NULL || ptr2 != NULL) {
            int val1 = ptr1 == NULL?0:ptr1->val;
            int val2 = ptr2 == NULL?0:ptr2->val;
            int sum = val1 + val2 + carry;
            val = sum;
            carry = sum/10;
            pre->next = new ListNode(val);
            pre = pre->next;
            if (ptr1!=NULL)
            {
                ptr1 = ptr1->next;
            }
            if (ptr2!=NULL) 
            {
                ptr2 = ptr2->next;
            }
        }
        if (carry == 1) {
            pre->next = new ListNode(1);
        }
        return result->next;
    }
};