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# 61. Rotate List

## 1.問題

* 給予一個list, 讓list旋轉k格

![](https://901207480-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LGKoChvN9am4__HCIRK%2F-LNbfaAKz0WhJWb-rltV%2F-LNbscmnEsc1uv3QUOj2%2F%E8%9E%A2%E5%B9%95%E5%BF%AB%E7%85%A7%202018-09-30%20%E4%B8%8A%E5%8D%888.30.30.png?alt=media\&token=22d8109a-d8f5-40c8-a4cf-c28030806fe2)

## 2.想法

* 提問
* parameter
* test input
* 觀察
  * 旋轉後將會讓第(n - k)個node為新的head
* 說明想法 
  * 先create新的ptr並移動到尾部, 讓list的尾部接到頭部, 形成cycle, 移動last向前 (n - k), 新的head即為last->next, 並讓last->next = NULL
* 測試計算複雜度

## 3.程式碼

```
/**
 * 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) {
        while (!head) {
            return head;
        }
        
        int index = 0, cnt = 1;
        ListNode* last = head, *newHead = 0;
        while (last->next) {
            last = last->next;
            cnt++;
        }
        last->next = head;

        k = (k % cnt);
        
        for (int i = 0 ; i < (cnt - k); i++) {
            last = last->next;
        }
        newHead = last->next;
        last->next = NULL;
        
        return newHead;
    }
};
```