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) {
        if(head1 == null || head2 == null){
            return head1 == null ? head2 : head1;
        }

        //两个都不为空，先反转，再相加，最后再反转回来
        ListNode revert1 = revert(head1);
        ListNode revert2 = revert(head2);

        int carry = 0;
        ListNode dummy = new ListNode(-1), cur = dummy;
        while(revert1 != null || revert2 != null){
            int val = (revert1 != null ? revert1.val : 0) + (revert2 != null ? revert2.val : 0) + carry;
            cur.next = new ListNode(val % 10);
            cur = cur.next;
            carry = val / 10;

            if(revert1 != null){
                revert1 = revert1.next;
            }
            if(revert2 != null){
                revert2 = revert2.next;
            }
        }

        //这个点要注意：如果最后了还有个进位1，一定也要拼上
        if(carry == 1){
            cur.next = new ListNode(1);
        }

        return revert(dummy.next);
    }

    private static ListNode revert(ListNode head){
        ListNode pre = null, cur = head;
        while(cur != null){
            ListNode next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }

        return pre;
    }
}