2-AddTwoNumbers.java

// 2 - Add Two Numbers

/**
 * 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 finalNode = new ListNode(0);
        ListNode finalHead = finalNode;
        int carry = 0;
        int sum = 0;
        
        while (l1 != null || l2 != null) {
          int l1Val = l1 != null ? l1.val :0;
          int l2Val = l2 != null ? l2.val :0;
          sum = l1Val + l2Val + carry;
            if (sum > 9) {
                carry = sum / 10;
                finalNode.next = new ListNode(sum % 10);
            } else {
                finalNode.next = new ListNode(sum % 10);
                carry = 0;
            }
            finalNode = finalNode.next;
            l1 = l1 != null ? l1.next : l1;
            l2 = l2 != null ? l2.next : l2;
        }     
        
        if (carry > 0) {
            finalNode.next = new ListNode(carry);
        }
        return finalHead.next;      
        
    }
}