链接：链表相加(二)_牛客题霸_牛客网 (nowcoder.com)

分析：

算法原理是逆序+高精度算法

逆序的原因是为了实现从低位（个位）开始相加。

public class Solution {
  
  //逆序链表
  public ListNode reverse(ListNode head){
    ListNode newHead = new ListNode(0);
    ListNode cur = head;
    while(cur != null){
      ListNode next = cur.next;
      cur.next = newHead.next;
      newHead.next = cur;
      cur = next;
    }
    return newHead.next;
  }
  public ListNode addInList (ListNode head1, ListNode head2) {
        //调用reverse()方法，以便从低位相加
        head1 = reverse(head1);
        head2 = reverse(head2);
        ListNode cur1 = head1;
        ListNode cur2 = head2;
        //t用来存储进位
        int t = 0;
        //ret作为头节点，prev初始化ret作为最终结果链表
        ListNode ret = new ListNode(0);
        ListNode prev = ret;
        while(cur1 != null || cur2 != null || t != 0){
          if(cur1 != null){
            t += cur1.val;
            cur1 = cur1.next;
          }
          if(cur2 != null){
            t += cur2.val;
            cur2 = cur2.next;
          }
          prev = prev.next = new ListNode(t % 10);
          t /= 10;
        }
        return reverse(ret.next);
    }
}