LeetCode解题报告:Reorder List

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

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

思路：

1.利用快慢两个指针将链表一分为二；

2.针对第二个子链表求倒序；

3.利用merge思想将两个子链表合并。

代码：

 public class ReorderSingleList {

     /**

      * Definition for singly-linked list. class ListNode { int val; ListNode

      * next; ListNode(int x) { val = x; next = null; } }

      */

     public void reorderList(ListNode head) {

         if (head == null || (head.next == null)) {

             return;

         }

         // fast and slow point find the mid position.

         ListNode fast = head;

         ListNode slow = head;

         while ((fast != null) && (fast.next != null)) {

             fast = fast.next.next;

             slow = slow.next;

         }

         // reverse the last second list.

         ListNode headnode = new ListNode(-1);

         headnode.next = slow;

         ListNode temp = headnode.next;

         while (temp.next != null) {

             ListNode insert = temp.next;

             ListNode currNext = insert.next;

             insert.next = headnode.next;

             headnode.next = insert;

             temp.next = currNext;

         }

         // merge insert

         ListNode firstcur = head;

         ListNode secondcur = headnode.next;

         while (firstcur != slow && (secondcur != slow)) {// at first,I make a mistake in here;

             ListNode firstnex = firstcur.next;

             ListNode secondnex = secondcur.next;

             firstcur.next = secondcur;

             secondcur.next = firstnex;

             firstcur = firstnex;

             secondcur = secondnex;

         }

     }

     public static void main(String[] args) {

         ListNode t5 = new ListNode(5);

         ListNode t4 = new ListNode(4, t5);

         ListNode t3 = new ListNode(3, t4);

         ListNode t2 = new ListNode(2, t3);

         ListNode t1 = new ListNode(1, t2);

         new ReorderSingleList().reorderList(t1);

         System.out.println(t1);

     }

 }