<p>Given a string <code>s</code>, remove duplicate letters so that every letter appears once and only once. You must make sure your result is <span data-keyword="lexicographically-smaller-string"><strong>the smallest in lexicographical order</strong></span> among all possible results.</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;= 10<sup>4</sup></code></li>
	<li><code>s</code> consists of lowercase English letters.</li>
</ul>

<p>&nbsp;</p>
<p><strong>Note:</strong> This question is the same as 1081: <a href="https://leetcode.com/problems/smallest-subsequence-of-distinct-characters/" target="_blank">https://leetcode.com/problems/smallest-subsequence-of-distinct-characters/</a></p>

class Solution:
  def removeDuplicateLetters(self, s: str) -> str:
    ans = []
    count = collections.Counter(s)
    used = [False] * 26

    for c in s:
      count[c] -= 1
      if used[string.ascii_lowercase.index(c)]:
        continue
      while ans and ans[-1] > c and count[ans[-1]] > 0:
        used[ord(ans[-1]) - ord('a')] = False
        ans.pop()
      ans.append(c)
      used[ord(ans[-1]) - ord('a')] = True

    return ''.join(ans)
16100