<p>Given a string <code>s</code>, return <em>the </em><span data-keyword="lexicographically-smaller-string"><em>lexicographically smallest</em></span> <span data-keyword="subsequence-string"><em>subsequence</em></span><em> of</em> <code>s</code> <em>that contains all the distinct characters of</em> <code>s</code> <em>exactly once</em>.</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;= 1000</code></li>
	<li><code>s</code> consists of lowercase English letters.</li>
</ul>

<p>&nbsp;</p>
<strong>Note:</strong> This question is the same as 316: <a href="https://leetcode.com/problems/remove-duplicate-letters/" target="_blank">https://leetcode.com/problems/remove-duplicate-letters/</a>
class Solution {
public:
    string smallestSubsequence(string s) {
        int n = s.size();
        string output;

        vector<bool>taken(26,false);
        vector<int>lastIndex(26,-1);
        stack<char> st;

        for(int i=0;i<n;i++){
            lastIndex[s[i] - 'a'] = i;
        }

        for(int i=0;i<n;i++){
            char curr = s[i];
            if(!taken[curr - 'a']){
                while (!st.empty() && st.top() > curr && lastIndex[st.top() - 'a'] > i) {

                    taken[st.top() - 'a'] = false;
                    st.pop();
                }
                st.push(curr);
                taken[curr - 'a'] = true;
            }
        }

        while(!st.empty()){
            output.push_back(st.top());
            st.pop();
        }

        reverse(output.begin(), output.end());
        return output;
    }
};
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