#Leetcode# 61. Rotate List

https://leetcode.com/problems/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

代码：

/**
 * 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) return NULL;
        ListNode* root = head;
        int len = 0;
        while(root) {
            root = root -> next;
            len ++;
        }
        k = k % len;
        if(k == 0) return head;
        ListNode* slow = head;
        ListNode* fast = head;
        for(int i = 0; i < k; i ++) {
            if(fast) fast = fast -> next;
        }
        if(!fast) return head;
        while(fast -> next) {
            slow = slow -> next;
            fast = fast -> next;
        }
        fast -> next = head;
        fast = slow -> next;
        slow -> next = NULL;
        return fast;
    }
};

　　要注意 k 的处理要 k % len 但是后面把两部分拼接的三句话不是很懂 :-(