Time: 133 ms (90.17%), Space: 22.7 MB (30.47%) - LeetHub

Author: karygauss03

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

@@ -0,0 +1,23 @@
+class Solution {
+public:
+    int countPalindromicSubsequence(string s) {
+        int n = s.size();
+        vector<vector<int>> ma(26);
+        for (int i = 0 ; i < n ; i++){
+            ma[s[i] - 'a'].push_back(i);
+        }
+        int ans = 0;
+        for (int i = 0 ; i < 26 ; i++){
+            if (ma[i].size() > 0){
+                for (int j = 0 ; j < 26 ; j++){
+                    int idx = upper_bound(ma[j].begin(), ma[j].end(), ma[i][0]) - ma[j].begin();
+                    if (idx != ma[j].size() && ma[j][idx] < ma[i].back()){
+                        ans++;
+                    }
+                }
+            }
+        }
+        return ans;
+        
+    }
+};