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) {
        // 先将链表反转，低位在前：1000->0001
        ListNode r1 = reverse(head1); 
        ListNode r2 = reverse(head2);
        int jw = 0; // 进位
        ListNode result =  null;
        while(r1!=null || r2!=null){ // 从低位开始相加
            int v1 = r1==null?0: r1.val;
            int v2 = r2==null?0: r2.val;
            int sum = v1 + v2 + jw;
            ListNode newNode = new ListNode(sum%10);
            newNode.next = result;
            result = newNode;
            jw = sum/10; // 计算进位
            if(r1!=null) r1 = r1.next;
            if(r2!=null) r2 = r2.next;
        }
        if(jw>0){ // 如果最后还有进位
            ListNode newNode = new ListNode(jw);
            newNode.next = result;
            result = newNode;
        }
        return result;
    }

    /**
     * 反转链表
     */
    private ListNode reverse(ListNode head){
        ListNode pre = null;
        while(head!=null){
            ListNode next = head.next;
            head.next = pre;
            pre = head;
            head = next;
        }
        return pre;
    }
}