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# 61. Rotate List

## problem description

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
```

## algorithm thought

可以看到其实就是将链表倒数第k个位置截断，然后拼接到前面。知道这个意思之后，就只需要考虑如果k大于size的问题了。如果k等于size，链表长度不变，这就很明显，我们可以令k=k%size。最后截断链表即可

## code

```cpp
/**
 * 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==NULL)
            return head;
        int size=1;
        ListNode* tail=head;
        while(tail->next){
            size++;
            tail=tail->next;
        }
        if(size==1)
            return head;
        k=k%size;
        if(k==0)
            return head;
        k=size-k-1;
        ListNode* pre=head;
        ListNode* cur=head->next;
        while(k--){
            pre=pre->next;
            cur=cur->next;
        }
        pre->next=NULL;
        tail->next=head;
        return cur;
    }
};
```

## algorithm analysis

开始算法得到链表的size，时间复杂度O(n)。然后找到倒数第k个节点，最后