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题目：（中等）

标签：贪心算法、栈、字符串

相关题目：与题目316相同

解法一（贪心算法）：

class Solution:    def smallestSubsequence(self, s: str) -> str:        if not s:            return ""        count = collections.Counter(s)        pos = 0        for i in range(len(s)):            if s[i] < s[pos]:                pos = i            count[s[i]] -= 1            if count[s[i]] == 0:                break        return s[pos] + self.smallestSubsequence(s[pos:].replace(s[pos], ""))

解法二（用栈维护最小字典序结果）：

class Solution:    def smallestSubsequence(self, s: str) -> str:        count = collections.Counter(s)        stack = []        for ch in s:            count[ch] -= 1            if ch not in stack:                while stack and stack[-1] > ch and count[stack[-1]] > 0:                    stack.pop()                stack.append(ch)        return "".join(stack)

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