Given a singly linked list L: L 0→L 1→…→L n-1→L n,
reorder it to: L 0→L n →L 1→L n-1→L 2→L n-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}.
/**
* 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
)
return
;
ListNode slow=head;
ListNode fast=head;
while
(fast.next!=
null
&&fast.next.next!=
null
){
slow=slow.next;
fast=fast.next.next;
}
ListNode preMiddle=slow;
ListNode precurr=slow.next;
while
(precurr.next!=
null
){
ListNode current=precurr.next;
precurr.next=current.next;
current.next=preMiddle.next;
preMiddle.next=current;
}
slow=head;
fast=preMiddle.next;
while
(slow!=preMiddle){
preMiddle.next=fast.next;
fast.next=slow.next;
slow.next=fast;
slow=fast.next;
fast=preMiddle.next;
}
}
}