题解 | #链表相加(二)#

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode prev = null;
        ListNode current = head;
        while (current != null) {
            ListNode next = current.next;
            current.next = prev;
            prev = current;
            current = next;
        }
        return prev;
    }


    public ListNode addInList(ListNode head1, ListNode head2) {
        ListNode l1,l2;
        l1 = reverseList(head1);
        l2 = reverseList(head2);

        ListNode dummyHead = new ListNode(0);
        ListNode current = dummyHead;
        int carry = 0;

        while (l1 != null || l2 != null) {
            int x = (l1 != null) ? l1.val : 0;
            int y = (l2 != null) ? l2.val : 0;
            int sum = carry + x + y;
            carry = sum / 10;
            current.next = new ListNode(sum % 10);
            current = current.next;
            if (l1 != null) l1 = l1.next;
            if (l2 != null) l2 = l2.next;
        }

        if (carry > 0) {
            current.next = new ListNode(carry);
        }

        return reverseList(dummyHead.next);
    }
}

首先先将两个链表进行反转，再相加。反转是常见的3节点，pre-cur-next,先改成cur指向pre，然后pre,cur,next依次向前移动。