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}.

基本思路：     用了一个很蠢很直接的方法，定位到中间节点，后半段入栈，然后把后半段与前半段拼接组成新链表

 /**
  * Definition for singly-linked list.
  * public class ListNode {
  *     int val;
  *     ListNode next;
  *     ListNode(int x) { val = x; }
  * }
  */
 public class Solution {
     public void reorderList(ListNode head) {

           if (head == null)
             return;

           ListNode node = head.next;
           int len = 0;
           while(node != null){
               len ++;
               node = node.next;
           }

           System.out.println("len:"+len);

           if(len == 2){
               ListNode temp = head.next;

               head.next = temp.next;
               temp.next = null;
               head.next.next = temp;
               return;
           }else if(len == 1 || len == 0){
               return;
           }

           int isOdd = len % 2;
           if(isOdd == 1){
               int mid = len / 2 + 1;
               Stack<ListNode> stack = new Stack();

               int j = 1;
               ListNode midNode = head.next;
               while(j != mid){
                   j++;
                   midNode = midNode.next;
               }

               ListNode realMidNode = midNode;
               midNode = midNode.next;
               j++;
               stack.push(midNode);
               while(j != len){
                   j++;
                   midNode = midNode.next;
                   stack.push(midNode);
               }

               j = 1;

               ListNode leftTemp = head.next;
               ListNode rightTemp = stack.pop();
               rightTemp.next = leftTemp;
               head.next = rightTemp;
                ListNode tail = leftTemp;
               j++;
               while(j < mid){
                   j++;
                   leftTemp = leftTemp.next;
                   rightTemp = stack.pop();
                   rightTemp.next = leftTemp;
                   tail.next = rightTemp;
                   tail = leftTemp;
               }

               tail.next = realMidNode;
               realMidNode.next = null;

           }else{
               int mid = len / 2 ;
               Stack<ListNode> stack = new Stack<>();

               int j = 1;
               ListNode midNode = head.next;
               while(j != mid){
                   j++;
                   midNode = midNode.next;
               }

               j++;
               midNode = midNode.next;
               stack.push(midNode);

               while(j != len){
                   j++;
                   midNode = midNode.next;
                   stack.push(midNode);
               }

               j=1;

               ListNode leftTemp = head.next;
               ListNode rightTemp = stack.pop();
               rightTemp.next = leftTemp;
               head.next = rightTemp;
                ListNode tail = leftTemp;  

               while(j != mid){
                   j++;
                   leftTemp = leftTemp.next;
                   rightTemp = stack.pop();
                   rightTemp.next = leftTemp;
                   tail.next = rightTemp;
                   tail = leftTemp;
               }
               tail.next = null;
           }

     }
 }

运行结果