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) {
        if(k<=0||head==NULL||head->next==NULL) return head;
        ListNode prehead(0);
        prehead.next=head;
        ListNode *p=head,*pre;
        int length=0;
        while(p)
        {
            length++;
            pre=p;
            p=p->next;
            
        }
        k=k%length;
        k=length-k;
        while(--k)
        {
            head=head->next;
        }
        
        pre->next=prehead.next;
        prehead.next=head->next;
        head->next=NULL;
        return prehead.next;
    }
};

下面是参考网上比较聪明的做法，利用环

/**
 * 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(k<=0||head==NULL||head->next==NULL) return head;
        ListNode prehead(0);
        prehead.next=head;
        ListNode *p=head;
        int length=0;
        while(p->next)
        {
            length++;
            p=p->next;
            
        }
        length++;
        k=k%length;
        k=length-k;
      p->next=head;
        while(k--)
        {
            p=p->next;
        }
        
        prehead.next=p->next;
        p->next=NULL;
        
        return prehead.next;
    }
};