<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();
        stack<char> st;
        vector<bool> seen(26,0);
        unordered_map<char,int> last_ind;
        for(int i=0;i<n;i++){
            last_ind[s[i]] = i;
        }

        for(int i=0;i<n;i++){
            
            if(seen[s[i] - 'a'] == 0){  
               while(!st.empty() && last_ind[st.top()] > i && st.top() > s[i]){
                 seen[st.top() - 'a'] =0;
                 st.pop();           
               }
               st.push(s[i]);
               seen[s[i] - 'a'] = 1;

            }

        }
        string result = "";
        while (!st.empty()) {
            result = st.top() + result;
            st.pop();
        }

        return result;
    }
};
9900