http://leetcode.com/onlinejudge#question_4

AddTwoNumbers.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        // Start typing your Java solution below
        // DO NOT write main() function
        int carry = 0;
        ListNode result = null, current = null;
        
        while(l1 != null || l2 != null || carry > 0) {
            int currentDigit = carry;

            if(l1 != null) {
                currentDigit += l1.val;
                l1 = l1.next;
            }
            if(l2 != null) {
                currentDigit += l2.val;
                l2 = l2.next;
            }
            
            ListNode newNode = new ListNode(currentDigit % 10);
            carry = currentDigit / 10;
            if(current == null) {
                result = newNode;
            } else {
                current.next = newNode;

            }
            current = newNode;
        }
        return result;
    }
}