Recently I came across a problem that asks to find out the non-unique characters in a string without any additional buffer. I interpreted this problem to be an in-place sort of the characters in the string and then iterating through it in order to track down the non-unique characters.
Another solution that can have O(1) space and O(n^2) run-time is having two 'for' loops going over the string to track down any pairs of characters that are common.
What I need is to sort the string in atleast O(nlogn) time with O(1) space.
Is there a simple/elegant way to do an in-place sort of characters in O(nlgn) with O(1) space?
There is a really nice comparison grid of sorting algorithms on wikipedia.
http://en.wikipedia.org/wiki/Sorting_algorithm#Comparison_of_algorithms
Heapsort and Smoothsort seem to be your obvious winners. Depending on your language you may or may not already have these, though I'm sure in most cases they're probably a library away.
There's a Java impl that at a glance claims to heapsort a set of anything Comparable.
http://www.java-tips.org/java-se-tips/java.lang/heap-sort-implementation-in-java.html