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  1. 分割字符串 ​

    有一个用逗号分隔的表示成绩的字符串，如

    "61,90,100,99,18,22,38,66,80,93,55,50,89"

    计算这些成绩的平均值

    c#
    string exStr = "61,90,100,99,18,22,38,66,80,93,55,50,89";
var res = exStr.Split(',').Select(s => Convert.ToInt32(s)).Average();
Console.WriteLine(res);
  2. 频率统计 ​

    统计一个字符串中每个字母出现的频率（忽略大小写）

    然后按照从高到低的顺序输出出现频率高于2次的单词和其出现的频率。

    "Hello world! My name is hah. hey huo"

    c#
    string exStr = "Hello world! My name is hah. hey huo";
var res = exStr.Where(c => char.IsLetter(c))
    .Select(c => char.ToLower(c))
    .GroupBy(c => c)
    .Select(g => new { g.Key, Count = g.Count() })
    .OrderByDescending(g => g.Count)
    .Where(g => g.Count > 2);
foreach(var item in res)
    Console.WriteLine($"key:{item.Key} \t count:{item.Count}");