public static void ReorderList(ListNode head){
if (head == null || head.next == null) //handle edge case
return;
//split the list use fast and slow pointers
ListNode fast = head;
ListNode slow = head;
while (fast.next != null && fast.next.next != null) {
fast = fast.next.next;
slow = slow.next;
}
ListNode second = slow.next;
slow.next = null; //final step for splitting list, cut two lists off
//reverse second list
second = ReverseList (second);
ListNode first = head;
while (first != null && second != null) {
ListNode temp1 = first.next;
ListNode temp2 = second.next;
second.next = first.next;
first.next = second;
first = temp1;
second = temp2;
}
}
private static ListNode ReverseList(ListNode head){ //reverse the list
if (head == null || head.next == null) //handle edge case
return head;
ListNode current = head;
ListNode next = head.next;
while (next != null) {
ListNode temp = next.next;
next.next = current;
current = next;
next = temp;
}
head.next = null; //set original head.next to null
return current;
}
public ListNode(int v){
this.val = v;
this.next = null;
}