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class Solution:
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    def removeDuplicateLetters(self, s: str) -> str:
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        last_occ = {}
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        stack = []
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        visited = set()
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        for i in range(len(s)):
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            last_occ[s[i]] = i
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        for i in range(len(s)):
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            if s[i] not in visited:
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                while stack and stack[-1] > s[i] and last_occ[stack[-1]] > i:
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                    visited.remove(stack.pop())
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                    stack.append(s[i])
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                    visited.add(s[i])
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            return "".join(stack)