Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode dummyHead = new ListNode(0);
        ListNode curr = dummyHead;
        int carry = 0;
        while (l1 != null || l2 != null) {
            int sum = 0;
            if (l1 == null && l2 != null) {
                sum = l2.val + carry;
                l2 = l2.next;
            } else if (l1 != null && l2 == null) {
                sum = l1.val + carry;
                l1 = l1.next;
            } else {
                sum = l1.val + l2.val + carry;
                l1 = l1.next;
                l2 = l2.next;
            }
            
            if (sum >= 10) {
                carry = 1;
                sum = sum - 10;
            } else {
                carry = 0;
                sum = sum;
            }
            
            ListNode res = new ListNode(sum);
            curr.next = res;
            curr = curr.next;            
        }
        
        if (carry == 1) {
            ListNode car = new ListNode(carry);
            curr.next = car;
            curr = curr.next;
        }        
        return dummyHead.next;
    }
}

Time Complexity : O(max[l1, l2])
Space Complexity : O(max[l1, l2])