Add two numbers II

// Add two numbers II - medium - 01/08/2023

// Approach 1 - by reversing ll
class Solution {
    private ListNode reverse(ListNode node){
     
        ListNode prev = null;
        ListNode next;

        while(node != null){
            next = node.next;
            node.next = prev;
            prev = node;
            node = next;
        }
        return prev;
    }

    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode ans = new ListNode();
        int sum = 0;
        int carry = 0;
        ListNode r1 = reverse(l1);
        ListNode r2 = reverse(l2);
        while(r1!=null || r2!= null){
            if(r1 != null) sum += r1.val;
            if(r2 != null) sum += r2.val;
            
            ans.val = sum % 10;
            carry = (sum)/10;
       
            ListNode head = new ListNode(carry);
            head.next = ans;
            ans = head;
            sum = carry;
            r1 = r1 != null ? r1.next : null;
            r2 = r2 != null ? r2.next : null;
        }
        
        return carry == 0? ans.next: ans;
    }
}

// Approach 2 - using stack

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 totalsum = 0;
        int carry = 0;
        ListNode ans = new ListNode();
        while(!s1.isEmpty() || !s2.isEmpty()){
            if(!s1.isEmpty())totalsum += s1.pop();
            if(!s2.isEmpty())totalsum += s2.pop();

            ans.val = (totalsum % 10);
            carry = totalsum / 10;
            ListNode newNode = new ListNode(carry);
            newNode.next = ans;
            ans = newNode;
            totalsum = carry;
        }
        if(carry == 0){
            return ans.next;
        }
        return ans;
    }
}