Rotate List

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

Example: 

Given 1->2->3->4->5->NULL and k = 2,

return 4->5->1->2->3->NULL.

思路：利用一个快慢指针，找到链表的结尾处，将链表的截尾与头相连形成环，将头指针移动相应的步数。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* rotateRight(ListNode* head, int k) {
       if(head == NULL || k <= 0)
            return head;
        int len = 1;
        ListNode *tail = head->next;
        ListNode *tail_pre = head;
        while(tail != NULL)//找到尾部
        {
            len++;
            tail = tail->next;
            tail_pre = tail_pre->next;
        }
       
        tail_pre->next = head;
        int steps = len - (k%len);
        //移动头指针
        while(steps > 0)
        {
            head = head->next;
            tail_pre = tail_pre->next;
            steps--;
        }
        //处理尾部
        tail_pre->next = NULL;
        return head;
        
    }
};