2.两数相加

    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode res = new ListNode(0);
        int num = 0;
        int carry = 0;
        ListNode current = res;
        while((l1 != null) || (l2 != null)) {            
            if(l1 != null && l2 != null) {
                num = l1.val + l2.val + carry;
                carry = 0;
                if(num > 9) {
                    carry = 1;
                    num = num % 10;
                }
                current.next = new ListNode(num);
                l1 = l1.next;
                l2 = l2.next;
                current = current.next;
            }else if(l1 == null) {
                if(carry == 0) {
                    num = l2.val;
                }else {                    
                    num = l2.val + carry;
                    carry = 0;
                    if(num > 9) {
                        num = num % 10;
                        carry = 1;
                    }
                }
                current.next = new ListNode(num);
                l2 = l2.next;
                current = current.next;
            }else {
                if(carry == 0) {
                    num = l1.val;
                }else {                    
                    num = l1.val + carry;
                    carry = 0;
                    if(num > 9) {
                        num = num % 10;
                        carry = 1;
                    }
                }
                current.next = new ListNode(num);
                l1 = l1.next;
                current = current.next;
            }
        }
        if(carry == 1) {
　　　　　　　　　//检查最后一次进位
            current.next = new ListNode(1);
        }
        return res.next;
    }