Add Two Numbers.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;      
 *     }
 * }
 */
public class Solution {
    /**
     * @param l1: the first list
     * @param l2: the second list
     * @return: the sum list of l1 and l2 
     */
    public ListNode addLists(ListNode l1, ListNode l2) {
        // write your code here
        ListNode dummy = new ListNode(0);
        ListNode curr = dummy;
        int count = 0;
        
        while (l1 != null || l2 != null) {
            int val1 = (l1 == null) ? 0 : l1.val;
            int val2 = (l2 == null) ? 0 : l2.val;
            int temp = val1 + val2 + count;
            if (temp > 9) {
                ListNode newNode = new ListNode(temp - 10);
                count = 1;
                curr.next = newNode;
            } else {
                ListNode newNode = new ListNode(temp);
                count = 0;
                curr.next = newNode;
            }
            curr = curr.next;
            l1 = (l1 == null) ? null : l1.next;
            l2 = (l2 == null) ? null : l2.next;
        }
        
        if (count == 1) {
            ListNode newNode = new ListNode(count);
            curr.next = newNode;
            curr = curr.next;
        }
        
        return dummy.next;
    }
}