143. Reorder List Leetcode Java

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}.

Solution:

3 steps:

1.Cut the list to two halves.

2.Reverse the second half of the list

3.Insert the second half nodes to first half of the list

Time complexity: O(n) in place

 public void reorderList(ListNode head) {  
     if(head==null || head.next==null) return;  
     ListNode walker=head;  
     ListNode runner=head;  
     while(runner!=null && runner.next!=null){  
       walker=walker.next;  
       runner=runner.next.next;  
     }  
     ListNode tail=walker.next;  
     if(tail==null) return;   
     while(tail.next!=null){  
       ListNode cur=tail.next;  
       tail.next=cur.next;  
       cur.next=walker.next;  
       walker.next=cur;  
     }  
     ListNode pre=head;  
     while(walker.next!=null){  
       ListNode cur=walker.next;  
       walker.next=cur.next;  
       cur.next=pre.next;  
       pre.next=cur;  
       pre=cur.next;  
     }  
   }