You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list. http://www.leetcode.com/onlinejudge

leetcode_add_two_numbers.cpp

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    // pay attention to pointer and pointer to pointer
    void update(ListNode** ret, ListNode** cur, int val) {
        ListNode* tmp = new ListNode(val);
        if (*ret == NULL) {
            *ret = tmp;
        }
        else {
            (*cur)->next = tmp;
        }
        *cur = tmp;
    }
    ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
        ListNode* ret = NULL;
        ListNode* cur = NULL;
        int addon = 0;
        while (l1 != NULL || l2 != NULL) {
            if (l1 != NULL) {
                addon += l1->val;
                l1 = l1->next;
            }
            if (l2 != NULL) {
                addon += l2->val;
                l2 = l2->next;
            }
            update(&ret, &cur, addon % 10);
            addon /= 10;
        }
        if (addon != 0) {
            update(&ret, &cur, addon % 10);
        }
        return ret;
    }
};