#1

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
```

#2

The below code uses a Stack to avoid reversing the list and adds them up.

``````class Solution {
public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
if(l1==null) return l2;
else if(l2==null) return l1;
Stack<ListNode> s1 = new Stack<>();
Stack<ListNode> s2 = new Stack<>();
ListNode t1=l1, t2=l2;
while(t1!=null){ s1.push(t1); t1=t1.next;}
while(t2!=null){ s2.push(t2); t2=t2.next;}
ListNode res = new ListNode(0);
int sum=0;
while(!s1.isEmpty() || !s2.isEmpty()){
if(!s1.isEmpty()){
sum += s1.pop().val;
}
if(!s2.isEmpty()){
sum += s2.pop().val;
}
ListNode temp = new ListNode(sum%10);
sum /= 10;
temp.next =res.next;
res.next = temp;
}
if(sum!=0){
ListNode temp = new ListNode(sum);
temp.next =res.next;
res.next = temp;
}
return res.next;
}
}
``````