<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 str) {
        int n=str.size();
        unordered_map<char,bool> taken;
        unordered_map<char,int> last_indx;
        for(int i=n-1;i>=0;i--){
            if (last_indx.find(str[i]) == last_indx.end()) {
                last_indx[str[i]] = i;
            }
        }

        string ans;
        for(int i=0;i<n;i++){
            if(taken[str[i]]) continue;

            while(!ans.empty() && ans.back()>str[i] && last_indx[ans.back()]>i){
                taken[ans.back()]=false;
                ans.pop_back();
            }
            ans.push_back(str[i]);
            taken[str[i]]=true;
        }

        return ans;
    }
};
9000