61-旋转链表

public ListNode rotateRight(ListNode head, int k) {
    if (head == null || head.next == null) return head;
    ListNode pre = head, post = head;
    while (k-- != 0){
        pre = pre.next != null ? pre.next : head;
    }
    while (pre.next != null){	//前后一起移动找到分割点post
        pre = pre.next;
        post = post.next;
    }
    pre.next = head;
    head = post.next;
    post.next = null;
    return head;
}

public ListNode rotateRight(ListNode head, int k) {
    if (head == null || head.next == null) return head;
    ListNode pre = head, post = head;
    int count = 1;
    while (pre.next != null){	//遍历1次获得链表长度
        pre = pre.next;
        count++;
    }
    k = k % count;	//计算位置
    pre = head;
    while (k-- != 0) pre = pre.next;	//就不用头尾循环了
    while (pre.next != null){
        pre = pre.next;
        post = post.next;
    }
    pre.next = head;
    head = post.next;
    post.next = null;
    return head;
}