Reorder List

题目：

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

 

 

代码：为何被注释掉的部分不对。

 

public class Solution {
    public void reorderList(ListNode head) 
    {
        if(head==null||head.next==null) return;
        
        ListNode slow = head;
        ListNode fast = head;
        ListNode firsthalf = head;
        
        while(fast!=null&&fast.next!=null&&fast.next.next!=null)
        {
            slow = slow.next;
            fast = fast.next.next;
        }
        
        ListNode secondhalf = slow.next;
        slow.next = null;
        
        secondhalf = reverseList(secondhalf);
        ListNode result = new ListNode(0);
        
        boolean flag = false;
       /* while (secondhalf.next!=null&firsthalf.next!=null)
        {
        if (flag == false)
        {
            result.next=firsthalf;
            firsthalf=firsthalf.next;
            flag=true;
        }
        if(flag == true)
        {
            result.next=secondhalf;
            secondhalf=secondhalf.next;
            flag=false;
        }
        } */
        
        while (secondhalf!= null&&firsthalf!=null) 
        {
            ListNode temp1 = firsthalf.next;
            ListNode temp2 = secondhalf.next;
 
            firsthalf.next = secondhalf;
            secondhalf.next = temp1;        
 
            firsthalf  = temp1;
            secondhalf = temp2;
        }
        
    }
    
    public ListNode reverseList(ListNode head) 
    {
        if (head == null) return head;
        ListNode cur = head;
        ListNode pre = null;
        while (cur.next != null){
            ListNode nextNode = cur.next;
            cur.next = pre;
            pre = cur;
            cur = nextNode;
        }
        cur.next = pre;
        return cur;
    }
}