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) {
        // Note: The Solution object is instantiated only once.
        if(head==NULL || k < 1) return head;

		ListNode* pre = head;
		int count = 1;
		while(pre->next && count <= k)
		{
			pre = pre->next;
			count++;
		}

		if(count == k)
			return head;
		else if(count < k)
		{
			k = k % count;
			if(k == 0)return head;
			pre=head;
			int num = 1;
			while(num < count - k)
			{
				pre = pre->next;
				num++;
			}
			ListNode* tmp = pre;
			while(pre->next)
				pre=pre->next;
			pre->next = head;
			head = tmp->next;
			tmp->next = NULL;
			return head;
		}else if(count == k+1)
		{
			ListNode* post = head;
			while(pre->next)
			{
				pre=pre->next;
				post=post->next;
			}
			pre->next = head;
			head = post->next;
			post->next = NULL;
			return head;
		}
    }
};