You are given two non-empty linked lists representing two non-negative integers. 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.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.


Solution:

Use a helper function (with additional carry argument) to recursively add nodes and move to next for l1 and l2.

/**
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* helper(ListNode* l1, ListNode *l2, int carry){
if(l1==NULL&&l2==NULL){
return carry?new ListNode(1):NULL;
}
int val = carry;
if(l1){
val += l1->val;
l1 = l1->next;
}
if(l2){
val += l2->val;
l2 = l2->next;
}
carry = val>=10;
if(carry) val -= 10;
ListNode *root = new ListNode(val);
root->next = helper(l1,l2,carry);
return root;
}
ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
return helper(l1,l2,0);

}
};


In fact no need to use recursion: just use while loop.