题解 | #链表相加(二)#

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
     */
    public ListNode addInList (ListNode head1, ListNode head2) {

        // 空处理
        if(head1 == null){
            return head2;
        }else if(head2 == null){
            return head1;
        }

        // 首先将链表反转
        head1 = reverse(head1);
        head2 = reverse(head2);

        // 这里专门创建一个链表存储新数据
        ListNode head = new ListNode(-1);
        ListNode fllowHead = head;

        // 进位数据
        int carry = 0;

        // 进行运算(这里当两者都不存在了再结束，在内部判断是否存在)
        while(head1 != null || head2 != null){

            // 首先加进位(result是初始值)
            int result = carry;

            // 判断head1
            if(head1 != null){
                result += head1.val;
                head1 = head1.next;
            }

            // 判断head2
            if(head2 != null){
                result += head2.val;
                head2 = head2.next;
            }

            // 运算并赋值
            carry = result / 10;
            fllowHead.next = new ListNode(result % 10);
            fllowHead = fllowHead.next;

        }

        // 当双链走完，仍有进位，需要加上
        if(carry == 1){
                fllowHead.next = new ListNode(1);
            }

        return  reverse(head.next);

    }

    // 反转链表
    public ListNode reverse(ListNode head){

        if(head == null){
            return null;
        }

        ListNode preNode = null;
        ListNode nextNode = null;

        while(head != null){

            nextNode = head.next;

            head.next = preNode;

            preNode = head;

            head = nextNode;
        }

        return preNode;

    }

    // 输出函数，调试
    public void printList(ListNode head){
        while(head != null){
            System.out.println(head.val);
        }
    }
}