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;    }}