Create 1930. Unique Length-3 Palindromic Subsequences1 (#681)

Author: Chayandas07

1930. Unique Length-3 Palindromic Subsequences1

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+class Solution {
+public:
+    int countPalindromicSubsequence(string s) {
+        vector<string>subbstr;
+        vector<int>a(26, 0); 
+        int n = s.size();
+        for(int i = 0; i < n; i++){
+            if(a[s[i] - 'a'] == 0){
+                string sub = s.substr(i, n - i);
+                subbstr.push_back(sub);
+                a[s[i] - 'a'] = 1;
+            }
+        }
+        int res = 0;
+        for(int i = 0; i < subbstr.size(); i++){
+            a.clear();
+            a.resize(26, 0);
+            int j = subbstr[i].size() - 1;
+            while(j > 0 && subbstr[i][j] != subbstr[i][0]){
+                j--;
+            }
+            j -= 1;
+            while(j > 0 ){
+                if(a[subbstr[i][j] - 'a'] == 0){
+                    res++;
+                    a[subbstr[i][j] - 'a'] = 1;
+                }
+                j--;
+            }
+        }
+        return res;
+    }
+};