<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) {
        int li[26]{0};
        for (int i = 0;i <s.size(); ++i) {
            li[s[i] - 'a'] = i;
        }
        bool seen[26]{false};
        stack<char> st;
        for (int i = 0;i <s.size(); ++i) {
            int c = s[i] - 'a';
            if (seen[c]) continue;
            while (!st.empty() && st.top() > s[i] && i <li[st.top() - 'a']) {
                seen[st.top() - 'a'] = false;
                st.pop();
            }
            st.push(s[i]);
            seen[c] = true;
        }
        string result;
        while (st.size()) {
            result.push_back(st.top());
            st.pop();
        }
        reverse(result.begin(), result.end());
        return result;
    }
};
8600