/*
 * @lc app=leetcode id=143 lang=java
 *
 * [143] Reorder List
 */

// @lc code=start
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public void reorderList(ListNode head) {
        if(head == null || head.next == null) {
            return;
        }
        // reverse the right side and merge
        ListNode d1 = new ListNode(0);
        d1.next = head;
        ListNode slow = d1, fast = d1;
        while(fast!=null && fast.next!=null) {
            fast = fast.next.next;
            slow = slow.next;
        }
        ListNode rh = slow.next, lh = head;
        slow.next = null;
        // reverse the right half
        ListNode prev = null, cur = rh, t;
        while(cur != null) {
            t = cur.next;
            cur.next = prev;
            prev = cur;
            cur = t;
        }
        rh = prev;
        // merge the two linkedlist
        while(rh != null) {
            t = lh.next;
            lh.next = rh;
            lh = rh;
            rh = t;
        }
    }
}
// @lc code=end