leetcode1081

找到一个跟我的思路比较接近的参考解决方案：

class Solution:
    def smallestSubsequence(self, text: str) -> str:
        n = len(text)
        d = collections.defaultdict(collections.deque)
        # record index
        for i,v in enumerate(text):
            d[ord(v)-ord('a')].append(i)
        
        # search orderly
        keys = sorted(d.keys())
        res = []
        c = len(d)
        last_index = -1
        while len(res) < c: # len(res) will not larger than 26
            # O(26*26) time, search the first smallest letter
            for i in range(len(keys)):
                if all(d[keys[i]][0] < d[keys[j]][-1] for j in range(i+1, len(keys))):
                    res.append(chr(ord('a')+keys[i]))
                    last_index = d[keys[i]][0]
                    keys.remove(keys[i])
                    break
            # O(n) time, delete all index less than last_index
            for i in range(len(keys)):
                while d[keys[i]] and d[keys[i]][0] < last_index:
                    d[keys[i]].popleft()
  
        return ''.join(res)

参考：https://leetcode.com/problems/smallest-subsequence-of-distinct-characters/discuss/308222/show-my-thinking-process