2.Add Two Numbers.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0);  // Dummy node to build result list
        ListNode curr = dummy;             // Pointer to current node
        int carry = 0;

        // Traverse both lists
        while (l1 != null || l2 != null || carry != 0) {
            int sum = carry;

            if (l1 != null) {
                sum += l1.val;
                l1 = l1.next;
            }
            if (l2 != null) {
                sum += l2.val;
                l2 = l2.next;
            }

            carry = sum / 10;                  // Carry for next iteration
            curr.next = new ListNode(sum % 10); // Create new node with the digit
            curr = curr.next;                  // Move forward
        }

        return dummy.next; // Return head of the new linked list
    }
}