# C#LeetCode刷题之#541-反转字符串 II（Reverse String II）

Given a string and an integer k, you need to reverse the first k characters for every 2k characters counting from the start of the string. If there are less than k characters left, reverse all of them. If there are less than 2k but greater than or equal to k characters, then reverse the first k characters and left the other as original.

Input: s = “abcdefg”, k = 2

Output: “bacdfeg”

Restrictions:

The string consists of lower English letters only.
Length of the given string and k will in the range [1, 10000]

```public class Program {

public static void Main(string[] args) {
var s = "abcdefg";
var k = 2;

var res = ReverseStr(s, k);
Console.WriteLine(res);

s = "abcdefg";
k = 3;

res = ReverseStr2(s, k);
Console.WriteLine(res);

s = "abcdefg";
k = 8;

res = ReverseStr3(s, k);
Console.WriteLine(res);

}

private static string ReverseStr(string s, int k) {
if(k > s.Length) return new string(s.Reverse().ToArray());
var nums = Math.Ceiling(s.Length / (double)k);
var sb = new StringBuilder(s);
for(var i = 0; i < nums; i += 2) {
var start = i * k;
var end = Math.Min(s.Length - 1, start + k - 1);
while(start < end) {
var swap = sb[start];
sb[start] = sb[end];
sb[end] = swap;
start++;
end--;
}
}
return sb.ToString();
}

private static string ReverseStr2(string s, int k) {
var chars = s.ToCharArray();
var offset = 2 * k;
for(var i = 0; i < chars.Length; i += offset) {
if(i + k > chars.Length) Array.Reverse(chars, i, chars.Length - i);
else Array.Reverse(chars, i, k);
}
return new string(chars);
}

private static string ReverseStr3(string s, int k) {
var list = new List<string>();
for(var i = 0; i < s.Length; i += k) {
if(i + k < s.Length) list.Add(s.Substring(i, k));
}
return string.Join("", list.Select((v, i) => i % 2 == 0 ? new string(v.Reverse().ToArray()) : v));
}

}```

```bacdfeg