Reorder List

Reorder List

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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}.

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public class Solution {
    public void reorderList(ListNode head) {
        //思路：先找到链表的中间节点，用fast和slow指针即可得到，最终的slow为中间指针，把slow后面的链表
        //翻转reverse，然后与head到slow的链表合并即可得到最终给的结果
        
              if(head==null)
 return ;
 ListNode fast;
 Stack<ListNode> stack=new Stack<ListNode>();
     //ListNode preHead=head;
     ListNode afterHead=null;
 ListNode slow=fast=head;
 ListNode q=null;
 while(fast!=null&&fast.next!=null){
 slow=slow.next;
     fast=fast.next.next;
     }
     afterHead=slow.next;
     ListNode p=afterHead;
     slow.next=null;
     while(p!=null){
     stack.push(p);
     p=p.next;
     }
     p=head;
     ListNode temp;
     while(!stack.isEmpty()&&p!=null){
    q=stack.pop();
    q.next=p.next;
    p.next=q;
    p=q.next;   
    }
     //return head;
    }
}