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Remove Duplicate Letters

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Description

https://leetcode.com/problems/remove-duplicate-letters/

Solution

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class Solution {
public:
    string removeDuplicateLetters(string s) {
        
        vector<char> st;
        vector<int> counter(26);
        vector<bool> visited(26);
        for (int i = 0; i < s.size(); ++i) 
            counter[s[i]-'a'] += 1;
        
        for (int i = 0; i < s.size(); ++i) {
            --counter[s[i]-'a'];
             if (visited[s[i]-'a']) continue;
            while (!st.empty() && st.back() > s[i] 
                && counter[st.back()-'a'] > 0) {
                visited[st.back()-'a'] = false;
                st.pop_back();
            }
            st.push_back(s[i]);
            visited[st.back()-'a'] = true;
        }
        
        return string(st.begin(), st.end());
    }
    
    
};