题解 | #重排链表#

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
import java.util.*;
public class Solution {
    public void reorderList(ListNode head) {
        //递归出口：未插入链表不足三位，无需插入
        if(head == null)
            return;
        if(head.next == null)
            return ;
        if(head.next.next == null)
            return ;
        //寻找未排序链表的首元素、首元素的下一个元素、尾元素，并把尾元素插入到第一二个元素中间
        ListNode second = head.next;
        ListNode tail = second.next;
        ListNode pre = second;
        while(tail.next!=null){
            pre = tail;
            tail = tail.next;
        }
        //切断尾元素
        pre.next = null;
        head.next = tail;
        tail.next = second;
        //重定位未排序部分链表的首元素
        head = second;
        //递归插入
        reorderList(second);
    }
}