ReOrder List java实现

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}.

分析

本题思路如下：首先将题目所给单链表从中间分割为两个单链表，再将后半个单链表反向，最后合并两个单链表即可。

代码

public class ReorderList {      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;          }            // find the second half head          ListNode fast = head.next;          ListNode slow = head;          while (fast != null && fast.next != null) {              fast = fast.next.next;              slow = slow.next;          }            // reverse the second half          ListNode p = slow.next;          slow.next = null; // cut the first half          ListNode pPre = null;          ListNode pSuf = p.next;          while (p != null) {              pSuf = p.next;              p.next = pPre;              pPre = p;              p = pSuf;          }            // combine two halves          ListNode l1 = head;          ListNode l2 = pPre;          while (l1 != null && l2 != null) {              ListNode l1Next = l1.next;              ListNode l2Next = l2.next;              l1.next = l2;              l2.next = l1Next;              l1 = l1Next;              l2 = l2Next;          }      }  }