61. Rotate List

https://leetcode.com/problems/rotate-list/

观察到：倒数第k%length个节点就是新头节点，新头节点的前一个点就是新尾节点

corner case：当k % length = 0的时候，rotate之后还是和原链表一样，故无需变换

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode rotateRight(ListNode head, int k) {
        if(head == null || k == 0){
            return head;
        }
        int length = 1;
        ListNode tail = head;
        while(tail.next != null){
            length += 1;
            tail = tail.next;
        }
        if(k % length == 0){
            return head;
        }
        int newHeadIndex = length - k % length;
        
        ListNode newTail = head;
        ListNode newHead = head.next;
        while(newHeadIndex != 1){
            newTail = newTail.next;
            newHead = newHead.next;
            newHeadIndex -= 1;
        }
        tail.next = head;
        newTail.next = null;
        return newHead;
    }
}