Time: 228 ms (82.88%) | Memory: 13.3 MB (65.98%) - LeetSync

Author: sayandip20

2059-unique-length-3-palindromic-subsequences/unique-length-3-palindromic-subsequences.cpp

@@ -0,0 +1,14 @@
+class Solution {
+public:
+    int countPalindromicSubsequence(string s) {
+        int first[26] = {[0 ... 25] = INT_MAX}, last[26] = {}, res = 0;
+        for (int i = 0; i < s.size(); ++i) {
+            first[s[i] - 'a'] = min(first[s[i] - 'a'], i);
+            last[s[i] - 'a'] = i;
+        }
+        for (int i = 0; i < 26; ++i)
+            if (first[i] < last[i])
+                res += unordered_set<char>(begin(s) + first[i] + 1, begin(s) + last[i]).size();
+        return res;
+    }
+};