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class Solution {
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 public:
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  int countPalindromicSubsequence(string s) {
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    vector<pair<int, int>> v(26, {-1, -1}); // to store first occurance  and
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                                            // last occurance of every alphabet.
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    int n = s.length(); // size of the string
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    for (int i = 0; i < n; i++) {
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      if (v[s[i] - 'a'].first == -1)
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        v[s[i] - 'a'].first = i; // storing when alphabet appered first time.
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      else
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        v[s[i] - 'a'].second = i; // else whenever it appears again. So that the
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                                  // last occurrence will be stored at last.
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    }
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    int ans = 0;
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    for (int i = 0; i < 26; i++) { // traversing over all alphabets.
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      if (v[i].second != -1) { // only if alphabet occured second time.
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        unordered_set<char> st; // using set to keep only unique elements between the range.
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        for (int x = v[i].first + 1; x < v[i].second; x++)
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          st.insert(s[x]); // set keeps only unique elemets.
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        ans += ((int)st.size()); // adding number of unique elements to the answer.
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      }
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    }
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    return ans;
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  }
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};