public static void reorderList(ListNode head) {
if (head == null || head.next == null || head.next.next == null) return;
ListNode n = head;
int len = 1;
while (n.next != null) {
n = n.next;
len++;
}
ListNode partition = head;
for (int i = 0; i < len / 2 - 1; i++) partition = partition.next;
ListNode second = partition.next;
partition.next = null;
second = reverse(second);
ListNode first = head;
ListNode dummy = new ListNode(-1);
while (first != null && second != null) {
dummy.next = first;
first = first.next;
dummy.next.next = second;
second = second.next;
dummy = dummy.next.next;
}
dummy.next = first == null ? second : first;
}
public static void main(String[] args) {
ListNode l = new ListNode(1);
l.next = new ListNode(2);
l.next.next = new ListNode(3);
l.next.next.next = new ListNode(4);
l.next.next.next.next = new ListNode(5);
l.display();
reorderList(l);
l.display();
}
public static ListNode reverse(ListNode head) {
if (head == null || head.next == null) return head;
ListNode prev = null;
ListNode curr = head;
while (curr != null) {
ListNode next = curr.next;
curr.next = prev;
prev = curr;
curr = next;
}
return prev;
}