c# - Testing for repeated characters in a string

Question

I'm doing some work with strings, and I have a scenario where I need to determine if a string (usually a small one < 10 characters) contains repeated characters.

`ABCDE`  // does not contain repeats 
`AABCD`  // does contain repeats, ie A is repeated

I can loop through the string.ToCharArray() and test each character against every other character in the char[], but I feel like I am missing something obvious.... maybe I just need coffee. Can anyone help?

EDIT:

The string will be sorted, so order is not important so ABCDA => AABCD

The frequency of repeats is also important, so I need to know if the repeat is pair or triplet etc.

Answer 1

If the string is sorted, you could just remember each character in turn and check to make sure the next character is never identical to the last character.

Other than that, for strings under ten characters, just testing each character against all the rest is probably as fast or faster than most other things. A bit vector, as suggested by another commenter, may be faster (helps if you have a small set of legal characters.)

Bonus: here's a slick LINQ solution to implement Jon's functionality:

int longestRun =
    s.Select((c, i) => s.Substring(i).TakeWhile(x => x == c).Count()).Max();

So, OK, it's not very fast! You got a problem with that?!

:-)