<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:
    bool again(int i,stack<char>& st, string& s, int n){
        for(int j=i+1;j<n;j++){
            if(st.top()==s[j]){
                return true;
            }
        }
        return false;
    }
    string smallestSubsequence(string s) {
        stack<char> st;
        map<char,bool> mpp;
        int n = s.size();
        for(int i=0;i<n;i++){
            if(mpp[s[i]]==true){
                continue;
            }
            if(st.empty() || st.top()<s[i]){
                st.push(s[i]);
                mpp[s[i]]=true;
            }
            else{
                while(!st.empty() && st.top()>s[i] && again(i,st,s,n)){
                    mpp[st.top()] = false;
                    st.pop(); 
                }
                st.push(s[i]);
                mpp[s[i]] = true;
            }
        }
        string str = "";
        int x = st.size(); 
        for(int i=0;i<x;i++){
            str.push_back(st.top());
            st.pop();
        }
        int start=0;
        int end = x-1;
        while(start<end){
            swap(str[start],str[end]);
            start++;
            end--;
        }
        return str;
    }
};
9200