## 445. 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
``````

Thoughts:

2. Using a stack

``````/**
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
class Solution {
public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
ListNode n1 = reverse(l1), n2 = reverse(l2);
int carry = 0;
ListNode temp = n1, pre = n1;
while((n1 != null) || (n2 != null) || (carry !=0)){
int v1 = n1 == null? 0 : n1.val;
int v2 = n2 == null? 0 : n2.val;

if(n1 == null){
n1 = new ListNode((v1 + v2 + carry)%10);
pre.next = n1;
}
else {
n1.val = (v1 + v2 + carry)%10;
}
carry = (v1 + v2 + carry) /10;
pre = n1;
n1 = n1 == null? null : n1.next;
n2 = n2 == null? null : n2.next;
}

return reverse(temp);

}
private ListNode reverse(ListNode l1){
ListNode cur = l1.next;
ListNode pre = l1;
pre.next = null;
while(cur!= null){
ListNode next = cur.next;
cur.next = pre;
pre = cur;
cur = next;
}
return pre;
}
}
``````

Code: Using a stack

``````/**
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
class Solution {
public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
Stack<Integer> s1 = new Stack<>();
Stack<Integer> s2 = new Stack<>();
while(l1 != null){
s1.push(l1.val);
l1 = l1.next;
}

while(l2 != null){
s2.push(l2.val);
l2 = l2.next;
}

int sum = 0;
ListNode cur = new ListNode(0);
while(!s1.isEmpty() || !s2.isEmpty()){
if(!s1.isEmpty()) sum += s1.pop();
if(!s2.isEmpty()) sum += s2.pop();

cur.val = sum%10;