# Problem 189: Rotate Array

> <https://leetcode.com/problems/rotate-array/>

## 思路

\* 三步翻转法 ![](https://1241747088-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-Lpv9LvBSlFaukf_ALqh%2F-Lpv9NPw3Ji1X5Vk8CES%2F-Lpv9ynSlNPw51-3Pa_H%2FrotateArray.png?generation=1569729542858491\&alt=media)

```java
public class Solution {
    public void rotate(int[] nums, int k) {
        int len = nums.length;
        k = k % len;
        if (k > len) {
            return;
        }

        helper(nums, 0, len - k - 1);
        helper(nums, len - k, len - 1);
        helper(nums, 0, len - 1);
    }

    private void helper(int[] nums, int start, int end) {
        while (start < end) {
            swap(nums, start++, end--);
        }
    }

    private void swap(int[] nums, int a, int b) {
        int temp = nums[a];
        nums[a] = nums[b];
        nums[b] = temp;
    }
}
```

## 易错点

１. k 有可能比数组长

```java
k = k % len;
```

1. index 之间的关系

   ```java
   helper(nums, 0, len - k - 1);
   helper(nums, len - k, len - 1);
   helper(nums, 0, len - 1);
   ```