<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:
        hashmap = {}

        for i in range(len(s)):
            hashmap[s[i]] = i
        
        used = set()
        stack = []

        for i in range(len(s)):
            if s[i] in used:
                continue
            
            while stack and stack[-1] > s[i] and hashmap[stack[-1]] > i:
                used.remove(stack[-1])
                stack.pop()

            if s[i] not in used:
                stack.append(s[i])
                used.add(s[i])

        return ''.join(stack)


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