Leetcode: 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.

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(head == NULL || k <= 0)
            return head;
        int length = 0;
        ListNode *temp = head;
        while(temp)
        {
            length ++;
            temp = temp -> next;
        }
        k = k % length;
        
        ListNode *fast = head;
        ListNode *slow = head;
        int i = 1;
        for(;i <= k; i++)
        {
            fast = fast -> next;
        }
        for (;i < length; i++)
        {
            fast = fast -> next;
            slow = slow -> next;
        }
        fast -> next = head;
        head = slow -> next;
        slow -> next = NULL;
        
        return head;
        
    }
};