五笔的编码与解码

面试题：五笔的编码范围是a到y的25个字母，从1位到4位的编码，

如果将五笔的编码按字典序排序，形成数组如下：a, aa, aaa, aaaa, aaab, aaac, …, b, ba, baa, baaa, baab…yyyx, yyyy

其中a的索引是0，aa的索引是1，aaa的索引是2，aaaa的索引是3，以此类推：

1）、编写一个函数，输入是任意一个合法的字符串，输出这个字符串对应的索引；

2）、编写一个函数，输入是任意一个合法的索引，输出这个索引对应的字符串。
代码实现：

using System;

namespace cchoop
{
    class Program
    {

        public static void Main()
        {
            Console.WriteLine(GetWubiIndexByStr("a"));
            Console.WriteLine(GetWubiIndexByStr("aa"));
            Console.WriteLine(GetWubiIndexByStr("aaa"));
            Console.WriteLine(GetWubiIndexByStr("aaaa"));
            Console.WriteLine(GetWubiIndexByStr("aaab"));
            Console.WriteLine(GetWubiIndexByStr("b"));

            Console.WriteLine(GetWubiStrByIndex(0));
            Console.WriteLine(GetWubiStrByIndex(1));
            Console.WriteLine(GetWubiStrByIndex(2));
            Console.WriteLine(GetWubiStrByIndex(3));
            Console.WriteLine(GetWubiStrByIndex(4));
            Console.WriteLine(GetWubiStrByIndex(9));
        }

        public static int GetWubiIndexByStr(string wubiStr)
        {
            //权重
            int[] factor = new int[] { 1 + 25 + 25 * 25 + 25 * 25 * 25, 1 + 25 + 25 * 25, 1 + 25, 1 };
            int index = wubiStr.Length - 1;
            for (int i = 0; i < wubiStr.Length; i++)
            {
                index += factor[i] * (wubiStr[i] - 'a');
            }
            return index;
        }
        public static string GetWubiStrByIndex(int index)
        {
            //权重
            int[] factor = new int[] { 1 + 25 + 25 * 25 + 25 * 25 * 25, 1 + 25 + 25 * 25, 1 + 25, 1 };
            char[] arr = new char[4];

            int i = 0;
            while (index > 0)
            {
                arr[i] = (char)('a' + index / factor[i]);
                index = index % factor[i];
                index--;
                i++;
            }

            return new string(arr).TrimEnd();
        }
    }
}

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