Add Two Numbers【LeetCode】

题目：

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.Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)Output: 7 -> 0 -> 8
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解析思路：

1、链表1和链表2对位相加，需要考虑链表1和链表2是否有相同量的数字

2、相加之后的结果值赋予链表re，但赋值的时候会用到re=re.next，所以还需要用到一个链表返回

代码：

/** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { val = x; } * } */

public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode ListHead = new ListNode(0);
        ListNode re = ListHead;
        int prev=0;
        if(l1==null&&l2==null){
            return null;
        }
        while(l1!=null || l2!=null){
            if(l1==null){
                re.next = new ListNode((l2.val+prev)%10);
                prev=(l2.val+prev)/10;
            }
            if(l2==null){
                re.next = new ListNode((l1.val+prev)%10);
                prev=(l1.val+prev)/10;
            }
            if(l1!=null && l2!=null){
              re.next = new ListNode((l1.val+l2.val+prev)%10);  
              prev=(l1.val+l2.val+prev)/10;
            }
            if(l1 != null){
                l1=l1.next;
            }
            if(l2 != null){
                l2=l2.next;
            }
            re = re.next;
        }
        if(prev>0){
            re.next = new ListNode(prev);
        }
        return ListHead.next;
    }
}