【LeetCode】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

public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        if(l1==null)
            return l2;
        if(l2==null)
            return l1;
        ListNode root = new ListNode(0);
        ListNode re = root;
        int pre = 0;
        
        while(l1!=null&&l2!=null){
            int sum = l1.val+l2.val+pre;
            ListNode temp = new ListNode(sum%10);
            root.next=temp;
            root = temp;
            pre = sum/10;
            l1=l1.next;
            l2=l2.next;
        }
        while(l1!=null&&l2==null){
            int sum = l1.val+pre;
            ListNode temp = new ListNode(sum%10);
            root.next=temp;
            root=temp;
            pre = sum/10;
            l1=l1.next;
        }
        while(l2!=null&&l1==null){
            int sum = l2.val+pre;
            ListNode temp = new ListNode(sum%10);
            root.next=temp;
            root=temp;
            pre = sum/10;
            l2=l2.next;
        }
        if(pre!=0){
            ListNode temp = new ListNode(1);
            root.next=temp;
        }
        return re.next;
    }
}