61. Rotate List

Given a list, rotate the list to the right by k places, where k is non-negative.

For example:
Given 1->2->3->4->5->NULL and k = 2,
return 4->5->1->2->3->NULL.

先遍历列表，得到列表长度。用k%len得到实际的step。如果step=0，返回head。如果不为0，先用fast向后移step位，再用slow和fast同时后移直到fast为null。变换新的head，拼接前后sublist，得到rotate后的list。代码如下：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode rotateRight(ListNode head, int k) {
        if (head == null) {
            return head;
        }
        ListNode slow = head, fast = head;
        int len = 0;
        while (fast != null) {
            fast = fast.next;
            len ++;
        }
        int step = k % len;
        if (step == 0) {
            return head;
        }
        fast = head;
        while (step != 0) {
            fast = fast.next;
            step --;
        }
        while (fast.next != null) {
            fast = fast.next;
            slow = slow.next;
        }
        ListNode currhead = slow.next;
        slow.next = null;
        fast.next = head;
        return currhead;
    }
}