面试题27：单链表向右旋转k个节点

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

For example:
Given1->2->3->4->5->NULLand k =2,
return4->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) {
        if(head == nullptr || head->next == nullptr) return head;

        int length = 0;
        ListNode* p = head;
        while(p){
            length++;
            p = p->next;
        }
        k = k % length;
        if(k == 0) return head;
        int m = length - k;
        p = head;
        while(m > 1){
            p = p->next;
            m--;
        }
        ListNode* right = p->next;
        p->next = nullptr;
        p = right;
        while(p->next){
            p = p->next;
        }
        p->next = head;
        head = right;
        return head;
    }
};