## PowerShell - 141 ##

Reads text from a file named 'a'.

    $x=@{}
    gc a|%{[char[]]$_|group|%{$c=$_.name.tolower().trim()
    $n=$_.count;$x[$c]=($n,$x[$c])[$n-lt$x[$c]]}}
    ($x.Keys|sort|%{$_*$x[$_]})-join""