<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 {
public:
    string removeDuplicateLetters(string s) {
        string res;
        unordered_map<char, int> lastPosMap;
        for (int i = 0; i != s.size(); ++i) lastPosMap[s[i]] = i;
        unordered_set<char> seen;
        for (int i = 0; i != s.size(); ++i) {
            if (seen.count(s[i])) continue;
            while (!res.empty() && 
                   lastPosMap[res.back()] > i && 
                   s[i] < res.back()) {
                seen.erase(res.back());
                res.pop_back();
            }
            res += s[i];
            seen.insert(s[i]);
        }
        return res;
    }
};
9500