题目：

题解：

class Solution:
    def removeDuplicateLetters(self, s: str) -> str:
        vis = defaultdict(int)
        cnt = defaultdict(int)
        for ch in s: cnt[ch] += 1
        queue = []
        for ch in s:
            if vis[ch] == 0:
                while queue and queue[-1] > ch and cnt[queue[-1]]:
                    vis[queue.pop()] = 0
                if not queue or queue != ch: queue.append(ch)
                vis[ch] = 1
            cnt[ch] -= 1
        return "".join(queue)