public static void main(String args[]) {
        RemoveDuplicateMaintainingOrder rm = new RemoveDuplicateMaintainingOrder();
        System.out.println(rm.removeDuplicateLetters("cbacdcbc"));
    }
Date 02/29/2016
@author Tushar Roy

Given a string remove duplicates from the string maintaining order
and getting lexicographically smallest string.

Reference:
https://leetcode.com/problems/remove-duplicate-letters/
src/com/interview/stackqueue/RemoveDuplicateMaintainingOrder.java