61. Rotate List

Given a linked list, rotate the list to the right by k places, where k is non-negative.

Example 1:

Input: 1->2->3->4->5->NULL, k = 2
Output: 4->5->1->2->3->NULL
Explanation:
rotate 1 steps to the right: 5->1->2->3->4->NULL
rotate 2 steps to the right: 4->5->1->2->3->NULL
Example 2:

Input: 0->1->2->NULL, k = 4
Output: 2->0->1->NULL
Explanation:
rotate 1 steps to the right: 2->0->1->NULL
rotate 2 steps to the right: 1->2->0->NULL
rotate 3 steps to the right: 0->1->2->NULL
rotate 4 steps to the right: 2->0->1->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) {
        
    
    if (k==0) return head;
    if(head==NULL) return NULL;
  
        
    ListNode* p=head;
    ListNode* pre=head;
 
    int length=1;
    int rot;
   while(p->next) {p=p->next; length++;}
   p->next=head; //构成一个环
   rot=length>k?length-k-1:length-k%length-1;
      
   while(rot--) pre=pre->next;
   p=pre->next;
    cout<<p->val<<endl;
   pre->next=NULL;
        return p;
       
        
    }
};