# LeetCode Add Two Numbers 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.

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```

```class Solution {
public:
ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
stack<int> s1, s2;
while (l1) {
s1.push(l1->val);
l1 = l1->next;
}
while (l2) {
s2.push(l2->val);
l2 = l2->next;
}
ListNode* ans = new ListNode(1); // 预设好的最高位进位:-)
int sum = 0;
while (!s1.empty() || !s2.empty()) {
if (!s1.empty()) {
sum += s1.top();
s1.pop();
}
if (!s2.empty()) {
sum += s2.top();
s2.pop();
}
ListNode* tmp = new ListNode(sum%10);
tmp->next = ans->next;
ans->next = tmp;
sum /= 10; // 下一次的进位
}
if (sum)return ans;
else return ans->next;
}
};
```