143. Reorder List

Given a singly linked list L: L0→L1→…→Ln-1→Ln,
reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…

You must do this in-place without altering the nodes' values.

For example,
Given {1,2,3,4}, reorder it to {1,4,2,3}.

    

public void reorderList(ListNode head) {
    if(head==null)
        return;
    ListNode slow = head;
    ListNode fast = head;
    //找到链表的中点，如1 2 3 4 5 6 7 8，则slow是4
    while (fast.next != null && fast.next.next != null) {
        fast = fast.next.next;
        slow = slow.next;
    }
    //链表后半边逆序，5 6 7 8 变成 8 7 6 5
    ListNode secMiddle = slow; //secMiddle:4
    ListNode secCur = reverse(slow.next);
    secMiddle.next = secCur;

    slow = head;//slow:1
    fast = secCur;//fast:8
    while (slow != secMiddle) {
        secMiddle.next = fast.next;//4->7
        fast.next = slow.next;//8->2
        slow.next = fast;//1->8
        fast = secMiddle.next;//fast:7
        slow = slow.next.next;//slow:2，现在链表：1 8 2 3 4 7 6 5
    }
}

private ListNode reverse(ListNode head) {
    if (head == null)
        return null;
    ListNode dummy = new ListNode(-1);
    ListNode second = null;
    while (head != null) {
        second = head.next;
        head.next = dummy.next;
        dummy.next = head;
        head = second;
    }
    return dummy.next;
}