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 || head.next == null)
        return;
    //找到链表的中点，如1 2 3 4 5 6 7 8，则p1是4
    ListNode p1 = head;
    ListNode p2 = head;
    while (p2.next != null && p2.next.next != null) {
        p1 = p1.next;
        p2 = p2.next.next;
    }

    //链表后半边逆序，5 6 7 8 变成 8 7 6 5
    ListNode middle = p1; //middle 是4
    ListNode temp = p1.next; //temp 是5
    middle.next = reverse(temp);

    //达到最后排序目的
    p1 = head;
    p2 = middle.next;
    while (p1 != middle) {
        middle.next = p2.next; //另4->7
        p2.next = p1.next; //8->2
        p1.next = p2;//1->8
        p1 = p2.next;//让p1等于2
        p2 = middle.next; //p2等于7，现在链表1 8 2 3 4 7 6 5
    }
}

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