leetcode [143]Reorder List

Given a singly linked list L: L0→L1→…→Ln-1→Ln,
reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…

You may not modify the values in the list's nodes, only nodes itself may be changed.

Example 1:

Given 1->2->3->4, reorder it to 1->4->2->3.

Example 2:

Given 1->2->3->4->5, reorder it to 1->5->2->4->3.

题目大意：

给定链表，对链表进行重排序。使得 L0→Ln→L1→Ln-1→L2→Ln-2→…。

解法：

1.使用快慢指针找到中间节点

2.将后半部分链表翻转

3.将链表重新拼接

java:

class Solution {
    public void reorderList(ListNode head) {
        if(head==null) return;
        ListNode p=head,q=head;
        //找到链表的中间节点p
        while(q.next!=null&&q.next.next!=null) {
            p = p.next;
            q = q.next.next;
        }

        //将后半部分链表翻转
        ListNode preMiddle=p;
        ListNode pre=null;
        ListNode cur=p.next;
        while(cur!=null){
            ListNode next=cur.next;
            cur.next=pre;
            pre=cur;
            cur=next;
        }
        preMiddle.next=pre;

        //将链表重新拼接
        p=head;
        q=preMiddle.next;
        while(p!=preMiddle){
            preMiddle.next=q.next;
            q.next=p.next;
            p.next=q;
            p=q.next;
            q=preMiddle.next;
        }
    }
}