Remove Duplicate Characters from String

Design an algorithm and write code to remove the duplicate characters in a string without using any additional buffer.

Sample Input/Output:

String: programming

after removing duplicate chars: progamin

String: words

after removing duplicate chars: words

String: texts

after removing duplicate chars: texs

String: a

after removing duplicate chars: a

String: aa

after removing duplicate chars: a

String: ba

after removing duplicate chars: ba