1930-unique-length-3-palindromic-subsequences.cpp

class Solution {
public:
    int countPalindromicSubsequence(string s) {
        vector<pair<int, int>> v(26, {-1, -1});
        for (int i = 0; i < s.size(); i++) {
            if (v[s[i] - 'a'].first == -1) v[s[i] - 'a'].first = i;
            else v[s[i] - 'a'].second = i;
        }
        
        int res = 0;
        for (int i = 0; i < 26; i++) {
            if (v[i].second != -1) {
                unordered_set<char> tmp;
                for (int j = v[i].first + 1; j < v[i].second; j++) tmp.insert(s[j]);
                res += tmp.size();
            }
        }
        
        return res;
    }
};