题目描述

 

Given a singly linked list LL 0→L 1→…→L n-1→L n,
reorder it to: L 0→L n →L 1→L n-1→L 2→L n-2→…

You must do this in-place without altering the nodes' values.

For example,
Given{1,2,3,4}, reorder it to{1,4,2,3}.

 

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public void reorderList(ListNode head) {
        if(head == null){
            return;
        }else{
            if(head.next == null || head.next.next == null){
                return;
            }
        }
        ListNode fast = head;
        ListNode slow = head;
        while(fast.next != null && fast.next.next != null){
            fast = fast.next.next;
            slow = slow.next;
        }
        ListNode after = slow.next;
        slow.next = null;
        ListNode pre = null;
        while(after != null){
              ListNode next = after.next;
              after.next = pre;
              pre = after;
              after = next;
        }
        //反转过链表之后,after已经到了null,此时的pre才是新链表的头节点
           after = pre;
        while(head != null && after != null){
            ListNode headnext = head.next;
            ListNode afternext = after.next;
            head.next = after;
            head = headnext;
            after.next = head;
            after = afternext;
        }
      
    }
}