```
public class Solution {
public string ReverseStr(string s, int k) {
char[] arr = s.ToCharArray();
int n = arr.Length;
int i = 0;
while(i < n) {
int j = Math.Min(i + k - 1, n - 1);
swap(arr, i, j);
i = i + 2 * k;
}
return new string(arr);
}
private void swap(char[] arr, int l, int r) {
while (l < r) {
char temp = arr[l];
arr[l++] = arr[r];
arr[r--] = temp;
}
}
}
```