String deduplication, filter non-numeric or numeric

********************Remove ****************

Solution one: 

We use a built-in element deduplication method Distinct

string a = "abcabcd";
char[] b = a.ToArray().Distinct().ToArray();
string c = string.Join("", b);

Solution two:

ArrayList alist = new ArrayList();
char[] strs = a.ToArray();
for (int i = 0; i < strs.Length; i++)
{
    if (!alist.Contains(strs[i]))
    {
         alist.Add(strs[i]);
     }
}

****************Filter Numeric or Non-Numeric ****************

/// Remove the numbers in the   string 
public  static  string RemoveNumber( string key)  
{  
      return Regex.Replace(key, @"\d", "");  
}  


// Remove non-numbers in the string 
public  static  string RemoveNotNumber( string key)  
{  
    return Regex.Replace(key, @"[^\d]*", "");  
}