https://leetcode.com/problems/unique-length-3-palindromic-subsequences

medium/unique_length_3_palindromic_subsequences.py

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+# https://leetcode.com/problems/unique-length-3-palindromic-subsequences
+
+class Solution:
+    def countPalindromicSubsequence(self, s: str) -> int:
+        first, last = {}, {}
+        for i in range(len(s)):
+            if s[i] not in first:
+                first[s[i]] = i
+            last[s[i]] = i
+        palinedromes = set()
+        for ch in first:
+            i = first[ch]
+            j = last[ch]
+            if i + 1 < j:
+                middles = s[i + 1 : j]
+                for middle in middles:
+                    palinedromes.add(''.join([ch, middle, ch]))
+        return len(palinedromes)
+
+