LeetCode Linked List Medium 2. Add Two Numbers

Description

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You are given two non-empty linked lists representing two non-negative integers. 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.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example:

Input: ( ->  -> ) + ( ->  -> )
Output:  ->  ->
Explanation:  +  = .

题目描述：两个非空链表，代表两个非负整数。链表从尾部到头部每一个元素代表非负整数的每一位的值。技术这两个数的值，并把结果放到一个链表中。

我的思路：链表长度不限，所以不能转成int 或者long 进行计算。同时 每一位进行计算的时候，会有进位产生。下面是我的解法

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode AddTwoNumbers(ListNode l1, ListNode l2) {
        ListNode res = new ListNode();
        ListNode temp = res;
        bool flag = false;
        int sum = ;
        while(l1 != null && l2 != null){
            temp.next = new ListNode((l1.val + l2.val + sum)%);
            sum = (l1.val + l2.val + sum)/;
            l1 = l1.next;
            l2 = l2.next;
            temp = temp.next;

        }
        while(l1 !=null){
            temp.next = new ListNode((l1.val + sum)%);
            sum = (l1.val+sum)/;
            l1 = l1.next;
            temp = temp.next;
        }
        while(l2 !=null){
            temp.next = new ListNode((l2.val + sum)%);
            sum = (l2.val+sum)/;
            l2 = l2.next;
            temp = temp.next;
        }
        if(sum != )
            temp.next=new ListNode();
        return res.next;
    }
}