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;    }};