<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) {
        vector<int> lastIndex(26,0);

        vector<bool> seen(26,false);

        stack<char> st;
        for(int i = 0;i<s.length();i++){
            lastIndex[s[i] - 'a'] = i;
        }

        for(int i = 0;i<s.length();i++){
            int curr = s[i] - 'a';
            if(seen[curr]) continue;

            while(st.size() > 0 && s[i] < st.top() && i < lastIndex[st.top() - 'a']){
                seen[st.top() - 'a'] = false;
                st.pop();
            }

            st.push(s[i]);
            seen[s[i] - 'a'] = true;
        }

        string ans = "";
        while(st.size() > 0){
            ans += st.top();
            st.pop();
        }

        reverse(ans.begin(),ans.end());

        return ans;
    }

};
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