# I found it easier to think about using the rightmost position of each character.
# If we haven't added the character to the  string, and doing so would violate lexicographical order, remove all characters currently in the string that have a later # rightmost position than the current index. 
# This ensures that we have the smallest lexicographical ordering.
from collections import Counter, defaultdict

class Solution:
    def removeDuplicateLetters(self, s: str) -> str:
        # Last position in string
        last = {}
        for i in range(len(s) - 1, -1, -1):
            if s[i] not in last:
                last[s[i]] = i


        final = []
        seen = defaultdict(bool)
        for i, char in enumerate(s):
            # However, we should only do that processing when