add two numbers

public class {
  public ListNode addTwoNumbers(ListNode l1 , ListNode l2){
    if(l1 == null)
      return l2;
    if(l2 == null)
      return l1;
    ListNode list = new ListNode(0);	
    ListNode result = list;
    int carry = 0;
    while(l1 != null || l2 != null){	//"或"处理长度不相等的数组、链表、字符串很有好处
      int sum = 0;
      /*
      int l1 = (l1 == null) ? 0 : l1.val;
      int l2 = (l2 == null) ? 0 : l2.val;
      sum = l1 + l2 + carry;
      */
      if(l1 != null){
        sum += l1.val;
        l1 = l1.next;
      }
      if(l2 != null){
        sum += l2.val;
        l2 = l2.next;
      }
      result.next = new ListNode((sum + carry) % 10);//每次用到新的节点都需要申请空间
      result = result.next;		//记得新的链表指针往后挪
      carry = (sum + carry) / 10;
    }
    if(carry == 1)
      result.next = new ListNode(1);
    return list.next;
  }
}