<p>Given a string <code>s</code>, return <em>the </em><span data-keyword="lexicographically-smaller-string"><em>lexicographically smallest</em></span> <span data-keyword="subsequence-string"><em>subsequence</em></span><em> of</em> <code>s</code> <em>that contains all the distinct characters of</em> <code>s</code> <em>exactly once</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;bcabc&quot;
<strong>Output:</strong> &quot;abc&quot;
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;cbacdcbc&quot;
<strong>Output:</strong> &quot;acdb&quot;
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s.length &lt;= 1000</code></li>
	<li><code>s</code> consists of lowercase English letters.</li>
</ul>

<p>&nbsp;</p>
<strong>Note:</strong> This question is the same as 316: <a href="https://leetcode.com/problems/remove-duplicate-letters/" target="_blank">https://leetcode.com/problems/remove-duplicate-letters/</a>
class Solution:
    def smallestSubsequence(self, s: str) -> str:
        n = len(s)
        bitmask = [0] * (n + 1)
        chars = defaultdict(list)
        for i in range(n - 1, -1, -1):
            char = s[i]
            bitmask[i] = bitmask[i + 1] | (1 << (ord(char) - ord('a')))
            chars[char].append(i)
        
        m = len(chars)
        for char in chars.keys():
            chars[char].reverse()

        res = []
        idx = -1
        for _ in range(m):
            for char in sorted(chars.keys()):
                i = chars[char][bisect.bisect_left(chars[char], idx)]
                if bitmask[i].bit_count() == m:
                    res.append(char)
                    idx = i
                    for j in range(i + 1, n):
                        bitmask[j] |= (1 << (ord(char) - ord('a')))
                    del chars[char]
                    break
        
        return ''.join(res)
16600