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2\. Add Two Numbers

Medium

You are given two **non-empty** linked lists representing two non-negative integers. The digits are stored in **reverse order**, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

**Example 1:**

![](https://assets.leetcode.com/uploads/2020/10/02/addtwonumber1.jpg)

**Input:** l1 = [2,4,3], l2 = [5,6,4]

**Output:** [7,0,8]

**Explanation:** 342 + 465 = 807. 

**Example 2:**

**Input:** l1 = [0], l2 = [0]

**Output:** [0] 

**Example 3:**

**Input:** l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]

**Output:** [8,9,9,9,0,0,0,1] 

**Constraints:**

*   The number of nodes in each linked list is in the range `[1, 100]`.
*   `0 <= Node.val <= 9`
*   It is guaranteed that the list represents a number that does not have leading zeros.

To solve the Add Two Numbers problem in Java using a `Solution` class, we'll follow these steps:

1. Define a `ListNode` class to represent nodes in a linked list.
2. Define a `Solution` class with a method named `addTwoNumbers`.
3. Inside the `addTwoNumbers` method, traverse both input linked lists simultaneously:
   - Keep track of a carry variable to handle cases where the sum of two digits exceeds 9.
   - Calculate the sum of the current nodes' values along with the carry.
   - Update the carry for the next iteration.
   - Create a new node with the sum % 10 and attach it to the result linked list.
   - Move to the next nodes in both input lists.
4. After finishing the traversal, check if there is any remaining carry. If so, add a new node with the carry to the result.
5. Return the head of the result linked list.

Here's the implementation:

```java
class ListNode {
    int val;
    ListNode next;
    
    ListNode() {}
    
    ListNode(int val) {
        this.val = val;
    }
    
    ListNode(int val, ListNode next) {
        this.val = val;
        this.next = next;
    }
}

public class Solution {
    
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode dummyHead = new ListNode();
        ListNode curr = dummyHead;
        int carry = 0;
        
        while (l1 != null || l2 != null) {
            int sum = carry;
            if (l1 != null) {
                sum += l1.val;
                l1 = l1.next;
            }
            if (l2 != null) {
                sum += l2.val;
                l2 = l2.next;
            }
            curr.next = new ListNode(sum % 10);
            curr = curr.next;
            carry = sum / 10;
        }
        
        if (carry > 0) {
            curr.next = new ListNode(carry);
        }
        
        return dummyHead.next;
    }

    // Helper method to print a linked list
    public void printList(ListNode head) {
        ListNode curr = head;
        while (curr != null) {
            System.out.print(curr.val + " ");
            curr = curr.next;
        }
        System.out.println();
    }

    public static void main(String[] args) {
        Solution solution = new Solution();

        // Test cases
        ListNode l1 = new ListNode(2, new ListNode(4, new ListNode(3)));
        ListNode l2 = new ListNode(5, new ListNode(6, new ListNode(4)));
        ListNode result1 = solution.addTwoNumbers(l1, l2);
        System.out.print("Example 1 Output: ");
        solution.printList(result1);

        ListNode l3 = new ListNode(0);
        ListNode l4 = new ListNode(0);
        ListNode result2 = solution.addTwoNumbers(l3, l4);
        System.out.print("Example 2 Output: ");
        solution.printList(result2);

        ListNode l5 = new ListNode(9, new ListNode(9, new ListNode(9, new ListNode(9, new ListNode(9, new ListNode(9, new ListNode(9)))))));
        ListNode l6 = new ListNode(9, new ListNode(9, new ListNode(9, new ListNode(9))));
        ListNode result3 = solution.addTwoNumbers(l5, l6);
        System.out.print("Example 3 Output: ");
        solution.printList(result3);
    }
}
```

This implementation provides a solution to the Add Two Numbers problem using linked lists in Java.