q25_k个一组翻转链表

public class ListNode {
    int val;
    ListNode next;
    ListNode(int x) {
        val = x;
    }
}
/**
 * 先连接成环再找断点 o(n)
 */
public class Solution {
    public ListNode rotateRight(ListNode head, int k) {
        if (head == null) {
            return null;
        }
        if (head.next == null) {
            return head;
        }
        ListNode oldTail = head;
        int n;
        for (n = 1; oldTail.next != null; n++) {
            oldTail = oldTail.next;
        }
        oldTail.next = head;
        ListNode newTail = head;
        for (int i = 0; i < n - k % n - 1; i++) {
            newTail = newTail.next;
        }
        ListNode newHead = newTail.next;
        newTail.next = null;
        return newHead;
    }
}