C++ 46ms using vector and set O(n)

  • 0

    I prefer set over unordered_set or unordered_map since it doesn't need hashing
    -> faster

    int firstUniqChar(string s) {
            vector<int> c(26,0);
            set<int> uniq;
            //count all characters
            for(int i = 0; i < s.size() ; i++)
                c[s[i] - 'a']++;
            //insert characters with count of 1 into set
            for(int i = 0; i < c.size(); i++)
                if(c[i] == 1) uniq.insert(i + 'a');
            //return index of the first element erased from the set
            for(int i = 0; i < s.size(); i++)
                if(uniq.erase(s[i])) return i;
            return -1;

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