输入：(2 -> 4 -> 3) + (5 -> 6 -> 4)
输出：7 -> 0 -> 8
原因：342 + 465 = 807

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int val=0, ListNode next=null) {
 *         this.val = val;
 *         this.next = next;
 *     }
 * }
 */
public class Solution {
    public ListNode AddTwoNumbers(ListNode l1, ListNode l2) {
        //进位
        int curr = 0;
        ListNode res = new ListNode();
        var newNode = res;

        while(l1 != null || l2 != null){
            int l1val = 0;
            int l2val = 0;
            if(l1 != null){
                l1val = l1.val;
            }
            if(l2 != null){
                l2val = l2.val;
            }
            var tempVal = l1val + l2val + curr;
            curr = tempVal / 10;
            var rval = tempVal % 10;
            newNode.next = new ListNode(rval);
            newNode = newNode.next;
            //有的话则取
            l1 = l1?.next;
            l2 = l2?.next;
        }
        // 最后的进位
        if(curr != 0){
            newNode.next = new ListNode(1);
        }

        return res.next;

    }
}