链表相加【翻转链表法】

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 * }
 */

public class Solution {
    /**
     * 
     * @param head1 ListNode类 
     * @param head2 ListNode类 
     * @return ListNode类
     */
    //反转链表
    public ListNode ReverseList(ListNode pHead) {
        if(pHead == null)
            return null;
        ListNode cur = pHead;
        ListNode pre = null;
        while(cur != null){
            //断开链表，要记录后续一个
            ListNode temp = cur.next;
            //当前的next指向前一个
            cur.next = pre;
            //前一个更新为当前
            pre = cur;
            //当前更新为刚刚记录的后一个
            cur = temp;
        }
        return pre;
    }
    public ListNode addInList (ListNode head1, ListNode head2) {
         //任意一个链表为空，返回另一个
        if(head1 == null)
            return head2;
        if(head2 == null)
            return head1;
        //反转两个链表
        head1 = ReverseList(head1);
        head2 = ReverseList(head2);
        //添加表头
        ListNode res = new ListNode(-1);
        ListNode head = res;
        //进位符号
        int carry = 0;
        //只要某个链表还有或者进位还有
        while(head1 != null || head2 != null || carry != 0){
            //链表不为空则取其值
            int val1 = head1 == null ? 0 : head1.val;
            int val2 = head2 == null ? 0 : head2.val;
            //相加
            int temp = val1 + val2 + carry;
            //获取进位
            carry = temp / 10;
            temp %= 10;
            //添加元素
            head.next = new ListNode(temp);
            head = head.next;
            //移动下一个
            if(head1 != null)
                head1 = head1.next;
            if(head2 != null)
                head2 = head2.next;
        }
        //结果反转回来
        return ReverseList(res.next);
        
    }
}