/**
 * You are given two linked lists representing two non-negative numbers. The
 * digits are stored in reverse order and each of their nodes contain a single
 * digit. Add the two numbers and return it as a linked list.
 * 
 * Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
 * Output: 7 -> 0 -> 8
 * 
 * Tags: Linkedlist, Math
 */
class AddTwoNum {

    public static void main(String[] args) {
        
    }

    /**
     * Create a pre head pointer
     * Build list node one by one
     * Use sum to track the current sum of nodes, or node
     * Reset sum using sum /= 10
     * Note whether there is carry for last digit
     */
    public ListNdoe addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode c1 = l1;
        ListNode c2 = l2;
        ListNode pre = new ListNode(0); // set pre head
        ListNode d = pre;
        int sum = 0; // the sum of two nodes
        while (c1 != null || c2 != null) { // traverse longer list
            if (c1 != null) { // add one list 
                sum += c1.val;
                c1 = c1.next; // move on 
            }
            if (c2 != null) { // add another list
                sum += c2.val;
                c2 = c2.next; // move on 
            }
            // build next node
            d.next = new ListNode(sum % 10); // digit for current node
            sum /= 10; // carry
            d = d.next; 
        }
        if (sum == 1) d.next = new ListNode(1); // note that can have carry at the last digit
        return pre.next;
    }
    
    public class ListNode {
        int val;
        ListNode next;
        ListNode(int x) {
            val = x;
            next = null;
        }
    }
}

@@ -420,19 +420,21 @@
 reate a 
-pre
+dummy
  head po
@@ -753,19 +753,21 @@
 istNode 
-pre
+dummy
  = new L
@@ -785,19 +785,21 @@
  // set 
-pre
+dummy
  head%0A  
@@ -821,12 +821,22 @@
 d = 
-pre;
+dummy; //digit
 %0A   
@@ -1493,19 +1493,21 @@
  return 
-pre
+dummy
 .next;%0A 

Update implementation,.