316.py

# def removeDuplicateLetters(s):
#     for char in sorted(set(s)):
#         suffix = s[s.index(char):]
#         if set(s) == set(suffix):
#             return char + removeDuplicateLetters(suffix.replace(char, ''))
#     return ''


# 스택을 이용해서 풀어보면
import collections

def removeDuplicateLetters(s):
    counter, seen, stack = collections.Counter(s),set(),[]
    
    for char in s:
        counter[char] -= 1
        if char in seen:
            continue
        
        while stack and char < stack[-1] and counter[stack[-1]] >0:
            seen.remove(stack.pop())
        stack.append(char)
        seen.add(char)
    
    return ''.join(stack)


s = 'cbacdcbc'
print(removeDuplicateLetters(s))