Implement a sorting algorithm with no change.
Perl, 89 characters

sub t{return if!@_;my$c=shift;t(grep{$_ lt$c}@_),$c,t(grep{$_ ge$c}@_)}print t split//,&lt;&gt;


This exploits the fact that QuickSort is effectively equivalent to the recurrence relation:

sorted x = () if x is empty, otherwise
           sorted (elements in x less than x[0]) :: x[0] ::
           sorted (elements in x greater than or equal to x[0] except for x[0] itself)


Edit

I originally misread the question and thought I had to sort the input strings, not the characters within a string. Hence my solution grows from 80 to 89 characters.