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

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问题

给定一个字符串和一个整数 k，你需要对从字符串开头算起的每个 2k 个字符的前k个字符进行反转。如果剩余少于 k 个字符，则将剩余的所有全部反转。如果有小于 2k 但大于或等于 k 个字符，则反转前 k 个字符，并将剩余的字符保持原样。

输入: s = "abcdefg", k = 2

输出: "bacdfeg"

要求:

该字符串只包含小写的英文字母。
给定字符串的长度和 k 在[1, 10000]范围内。

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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]

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示例

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);

        Console.ReadKey();
    }

    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));
            else list.Add(s.Substring(i, s.Length - i));
        }
        return string.Join("", list.Select((v, i) => i % 2 == 0 ? new string(v.Reverse().ToArray()) : v));
    }

}

以上给出3种算法实现，以下是这个案例的输出结果：

bacdfeg
cbadefg
gfedcba

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分析：

显而易见，以上3种算法的时间复杂度均为  。