LeetCode Solutions : 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 itto {1,4,2,3}.

Considering the following steps:

 * 1. split such list into two list, first and second, according to slow and fast point
 * 2. reverse the second list
 * 3. insert the second list into the first list

coding solution:

/*** Definition for singly-linked list.* class ListNode {*     int val;*     ListNode next;*     ListNode(int x) {*         val = x;*         next = null;*     }* }* */
public class Solution {public void reorderList(ListNode head) {if(head!=null&&head.next!=null){ListNode low=head;//1. split such list into two list, first and second, according to slow and fast pointListNode fast=head;while(fast.next!=null&&fast.next.next!=null){low=low.next;fast=fast.next.next;}ListNode first=head;ListNode second=low.next;low.next=null;second=reverse(second);//2. reverse the second listwhile(second!=null){//3. insert the second list into the first listListNode p1=first.next;ListNode p2=second.next;first.next=second;second.next=p1;first=p1;second=p2;}}}private ListNode reverse(ListNode head){if(head==null||head.next==null)return head;ListNode pre=head;ListNode cur=head.next;while(cur!=null){ListNode nextNode=cur.next;cur.next=pre;pre=cur;cur=nextNode;			}head.next=null;return pre;}
}

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