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.

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

Thoughts:

1. Reversing the linkedList
2. Using a stack

Code: Reversing the LinkedList

/**
 * Definition for singly-linked list.
 * 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

/**
 * Definition for singly-linked list.
 * 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;
            ListNode head = new ListNode(sum/10);
            head.next = cur; // reconstruct
            cur = head;// moving on
            sum/=10;
        }
        return cur.val == 0? cur.next: cur;
    }
}