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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public void reorderList(ListNode head) {
        if (head == null || head.next == null) {
            return;
        }
        
        ListNode fast = head, slow = head;
        while (fast.next != null && fast.next.next != null) {
            fast = fast.next.next;
            slow = slow.next;
        }
        
        ListNode preMiddle = slow;
        ListNode curr = preMiddle.next;
        
        while (curr.next != null) {
            ListNode next = curr.next;
            curr.next = next.next;
            next.next = preMiddle.next;
            preMiddle.next = next;
        }
        
        ListNode node = head;
        curr = preMiddle.next;
        while (node != preMiddle) {
            preMiddle.next = curr.next;
            curr.next = node.next;
            node.next = curr;
            node = curr.next;
            curr = preMiddle.next;
        }
    }
}

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    private ListNode reverse(ListNode head) {
        ListNode newHead = null;
        while (head != null) {
            ListNode next = head.next;
            head.next = newHead;
            newHead = head;
            head = next;
        }
        return newHead;
    }
    public void reorderList(ListNode head) {
        if (head == null || head.next == null) return;
        ListNode fast = head, slow = head;
        while (fast.next != null && fast.next.next != null) {
            fast = fast.next.next;
            slow = slow.next;
        }
        fast = head;
        ListNode newHead = reverse(slow.next);
        slow.next = null;
        ListNode q = newHead;
        while (q != null) {
            ListNode next = q.next;
            q.next = fast.next;
            fast.next = q;
            fast = q.next;
            q = next;
        }
        return;
    }
}