<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) {
        stack<int>st;
        int n=s.size();
        set<char>observed;
        vector<int>last_ind(n,0);
        for(int i=n-1;i>=0;i--){
            if(!observed.count(s[i])){
                last_ind[i]=1;
                observed.insert(s[i]);
            }
        }
        int lockedIndex=-1;
        vector<int>indexInStack(26,-1);
        for(int i=0;i<n;i++){
            if(indexInStack[s[i]-'a']==-1){
               while(!st.empty() and st.top()>lockedIndex and s[st.top()]>s[i]){
                indexInStack[s[st.top()]-'a']=-1;
                st.pop();
               }
               st.push(i);
               indexInStack[s[i]-'a']=i;
            }

            if(last_ind[i]==1){
                lockedIndex=max(lockedIndex,indexInStack[s[i]-'a']);
            }
        }
        string ans="";
        while(!st.empty()){
            ans+=s[st.top()];st.pop();
        }
        reverse(ans.begin(),ans.end());
        return ans;
    }
};
8200