<p>Given a string <code>s</code>, remove duplicate letters so that every letter appears once and only once. You must make sure your result is <span data-keyword="lexicographically-smaller-string"><strong>the smallest in lexicographical order</strong></span> among all possible results.</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;= 10<sup>4</sup></code></li>
	<li><code>s</code> consists of lowercase English letters.</li>
</ul>

<p>&nbsp;</p>
<p><strong>Note:</strong> This question is the same as 1081: <a href="https://leetcode.com/problems/smallest-subsequence-of-distinct-characters/" target="_blank">https://leetcode.com/problems/smallest-subsequence-of-distinct-characters/</a></p>

class Solution {
public:
    string removeDuplicateLetters(string s) 
    {
        int n = s.size();
        unordered_map<char, int> last;
        for(int i = 0; i < n; i++)
            last[s[i]] = i;
        
        string st;  // Monotonic Increasing stack
        unordered_set<char> used;
        for(int i = 0; i < n; i++){
            while(st.size() && !used.count(s[i]) 
            && s[i] <= st.back() && last[st.back()] > i){
                used.erase(st.back());
                st.pop_back();
            }
            if(!used.count(s[i])){
                st.push_back(s[i]);
                used.insert(s[i]);
            }
        }
        return st;
    }
};
// For same character, the earlier used is better, ex: abacb
// used: bcda vs abcda
9600