leetcode Rotate List

/**
 * 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) {
        ListNode* fast = head, *slow = head;
        int len = 0;
        while(fast != NULL){
            ++len;
            fast = fast->next;
        }
        if(k == 0 || len == 0){
            return head;
        }
        k %= len;
        int cnt = 0;
        fast = head;
        while(cnt < k){
            fast = fast->next;
            ++cnt;
        }
        while(fast->next != NULL){
            fast = fast->next;
            slow = slow->next;
        }
        fast->next = head;
        head = slow->next;
        slow->next = NULL;
        return head;
    }
};