[Leetcode][JAVA] Reorder List

{1,2,3,4}

{1,4,2,3}

public void reorderList(ListNode head) {
if(head==null || head.next==null)
return;
//split
ListNode fast = head;
ListNode slow = head;
while(true)
{
fast = fast.next.next;
if(fast==null || fast.next==null)
break;
slow = slow.next;
}
fast = slow.next;
slow.next = null;
slow = head;
//reverse fast
fast = reverse(fast);
//insert
while(slow!=null && fast!=null)
{
ListNode snext=slow.next;
ListNode fnext=fast.next;
slow.next=fast;
if(snext!=null)
fast.next=snext;
slow = snext;
fast = fnext;
}
}
public ListNode reverse(ListNode head)
{
ListNode cur = head;
ListNode pre = null;
while(cur!=null)
{
ListNode temp = cur.next;
cur.next = pre;
pre = cur;
cur = temp;
}
return pre;
}