LeetCode 刷题笔录 Add Two Numbers

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

这题看起来没啥好说的，挨个顺序扫描就行了。要注意null的位置以及进位的问题。我的代码通不过测试。。。输入{9},{9}显示我的程序返回是{9}，{1}，而答案应该是{8}，{1}。但是我把这代码在自己机器上用eclipse编译完了答案是正确的。。。也看不出自己哪里有错，可能是leetcode编译的问题吧。先记下来再说。

private boolean carry = false;      //set the flag to true if a carry is needed 
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        // Start typing your Java solution below
        // DO NOT write main() function
        int first = add(l1.val, l2.val);
        ListNode previous = new ListNode(first);      //the first node of the answer linked list
        ListNode header = previous;
        for(l1 = l1.next, l2 = l2.next; l1 != null && l2 != null; l1 = l1.next, l2 = l2.next){
            previous = previous.next = new ListNode(add(l1.val, l2.val));
        }
        if(l1 == null && l2 == null){
            if(carry){
                previous = previous.next = new ListNode(1);
            }
        }
        else if(l1 == null){
            while(l2 != null){
                previous = previous.next = new ListNode(l2.val);
                l2 = l2.next;
            }
        }
        else if(l2 == null){
            while(l1 != null){
                previous = previous.next = new ListNode(l1.val);
                l1 = l1.next;
            }
        }
        
        return header;
    }
    /**
     * add the two numbers
     * return the last digit of the result of the sum of the two numbers
     * if the sum is larger than 10, set the carry flag to true
     */
    private int add(int a, int b){
        int result = a + b;
        if(carry){
            result++;
        }
        if(result < 10){
            carry = false;
            return result;
        }
        else{
            result = result % 10;
            carry = true;
            return result;
        }
    }