<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.length();
        string result;
        
        vector<bool> taken(26, false); //O(1) space
        vector<int> lastIndex(26); //O(1) space
        
        for(int i = 0; i < n; i++) {
            char ch = s[i];
            
            lastIndex[ch-'a'] = i;
        }
        
        
        for(int i = 0; i < n; i++) {
            
            int idx = s[i] - 'a';
            
            if(taken[idx] == true) continue;
            
            while(result.length() > 0 && s[i] < result.back() && lastIndex[result.back() - 'a'] > i) {
                taken[result.back() - 'a'] = false;
                result.pop_back();
            }
            
            result.push_back(s[i]);
            taken[idx] = true;
            
        }
        
        return result;
    }
};   

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