61. rotate list

 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class  {
public:
    ListNode* rotateRight(ListNode* head, int k) {
        if(!head)
            return head;
        ListNode* tail=head;
        int size=1;
        //get the size of the ListNode
        while(tail->next)
        {
            tail=tail->next;
            ++size;
        }
        tail->next=head;//circle the ListNode
        k%=size;//in order to avoid the repeated conduction
        //get the position to break the circle
        for(int i=0;i<size-k;++i)
        {
            tail=tail->next;
        }
        ListNode* ans=tail->next;
        //break the circle
        tail->next=NULL;
        return ans;
    }
};