addTwoNumbers.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {        
        ListNode p1 = l1, p2 = l2;
        ListNode result = new ListNode(-1);
        ListNode p3 = result;
        
        int carry = 0;
        
        while(p1 != null || p2!= null)
        {
            if(p1 != null)
            {
                carry += p1.val;
                p1 = p1.next;
            }
            
            if(p2 != null)
            {
                carry += p2.val;
                p2 = p2.next;
            }
            
            p3.next = new ListNode(carry%10);
            carry /= 10;
            p3 = p3.next;
        }
        
        if(carry == 1)
            p3.next = new ListNode(1);
        
        return result.next;
        
    }    
    
}