<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) {
        vector<int> freq(26,0);
        for(int i=0;i<s.size();i++){
            freq[s[i]-'a']++;
        }   
        stack<int> st;
        vector<bool> seen(26,false);
        for(int i=0;i<s.size();i++){
            if(seen[s[i]-'a']){
                freq[s[i]-'a']--;
                continue;
            }
            while(!st.empty() && st.top()>s[i] && freq[st.top()-'a']>0){
                seen[st.top()-'a']=false;
                st.pop();
            }
            st.push(s[i]);
            seen[s[i]-'a']=true;
            freq[s[i]-'a']--;
        }  
        string ans="";
        while(!st.empty()){
            ans.push_back(st.top());
            st.pop();
        }
        reverse(ans.begin(),ans.end());
        return ans;
    }
};
8400