Add Two Number II

You are given two non-empty linked lists representing two non-negative integers. The most significant digit comes first 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.

Follow up:

What if you cannot modify the input lists? In other words, reversing the lists is not allowed.

Example:

Input: (7 -> 2 -> 4 -> 3) + (5 -> 6 -> 4)

Output: 7 -> 8 -> 0 -> 7

/**
* 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) {
stack<int> ls1, ls2;
while (l1) {
ls1.push(l1->val);
l1 = l1->next;
}
while (l2) {
ls2.push(l2->val);
l2 = l2->next;
}
int carry = 0; ListNode *r = NULL, *p = r;
while (!ls1.empty() || !ls2.empty() || carry) {
int a = ls1.empty() ? 0 : ls1.top(), b = ls2.empty() ? 0 : ls2.top();
int sum = a + b + carry;
p = new ListNode(sum % 10);
p->next = r;
r = p;
carry = sum / 10;
if (!ls1.empty()) ls1.pop();
if (!ls2.empty()) ls2.pop();
}
return r;
}
};