# C#LeetCode刷题之#189-旋转数组（Rotate Array）

• 尽可能想出更多的解决方案，至少有三种不同的方法可以解决这个问题。
• 要求使用空间复杂度为 O(1) 的原地算法。

Given an array, rotate the array to the right by k steps, where k is non-negative.

Input: [1,2,3,4,5,6,7] and k = 3

Output: [5,6,7,1,2,3,4]

Explanation:

rotate 1 steps to the right: [7,1,2,3,4,5,6]

rotate 2 steps to the right: [6,7,1,2,3,4,5]

rotate 3 steps to the right: [5,6,7,1,2,3,4]

Input: [-1,-100,3,99] and k = 2

Output: [3,99,-1,-100]

Explanation:

rotate 1 steps to the right: [99,-1,-100,3]

rotate 2 steps to the right: [3,99,-1,-100]

Note:

• Try to come up as many solutions as you can, there are at least 3 different ways to solve this problem.
• Could you do it in-place with O(1) extra space?

```public class Program {

public static void Main(string[] args) {
int[] nums = null;
//[1,2,3,4,5,6,7] k = 3
//第1次，反转尾部，1,2,3,4,7,6,5
//第2次，反转头部，4,3,2,1,7,6,5
//第3次，反转所有，5,6,7,1,2,3,4

nums = new int[] { 1, 2, 3, 4, 5, 6, 7 };
Rotate(nums, 3);
ShowArray(nums);

nums = new int[] { 1, 2, 3, 4, 5, 6, 7 };
Rotate2(nums, 3);
ShowArray(nums);

nums = new int[] { 1, 2, 3, 4, 5, 6, 7 };
Rotate3(nums, 3);
ShowArray(nums);

}

private static void ShowArray(int[] array) {
foreach(var num in array) {
Console.Write(\$"{num} ");
}
Console.WriteLine();
}

private static void Rotate(int[] nums, int k) {
int[] newNums = new int[k];
int length = nums.Length;
k %= length;
int mid = length - k;
for(int i = 0; i < k; i++) {
newNums[i] = nums[mid + i];
}
for(int i = mid - 1; i >= 0; i--) {
nums[i + k] = nums[i];
}
for(int i = 0; i < k; i++) {
nums[i] = newNums[i];
}
}

private static void Reverse(int[] array, int index, int length) {
for(int i = index, count = -1; i < index + length / 2; i++) {
count++;
int t = array[i];
array[i] = array[index + length - count - 1];
array[index + length - count - 1] = t;
}
}

private static void Rotate2(int[] nums, int k) {
if(nums.Length == 1 || k == 0) {
return;
}
k %= nums.Length;
Reverse(nums, nums.Length - k, k);
Reverse(nums, 0, nums.Length - k);
Reverse(nums, 0, nums.Length);
}

private static void Rotate3(int[] nums, int k) {
if(nums.Length == 1 || k == 0) {
return;
}
k %= nums.Length;
Array.Reverse(nums, nums.Length - k, k);
Array.Reverse(nums, 0, nums.Length - k);
Array.Reverse(nums);
}

}```

```5 6 7 1 2 3 4
5 6 7 1 2 3 4
5 6 7 1 2 3 4```

Rotate2的解法符合题目“原地打转”要求。