Time: 191 ms (57.59%), Space: 55.2 MB (5.5%) - LeetHub

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

@@ -0,0 +1,26 @@
+class Solution {
+    public int countPalindromicSubsequence(String s) {
+        HashMap<Character, ArrayList<Integer>> map = new HashMap<>();
+        for (int i = 0; i < s.length(); i++) {
+            map.putIfAbsent(s.charAt(i), new ArrayList<>());
+            map.get(s.charAt(i)).add(i);
+        }
+
+        int ans = 0;
+        for (HashMap.Entry<Character, ArrayList<Integer>> entry : map.entrySet()) {
+            ArrayList<Integer> indices = entry.getValue();
+            int start = indices.get(0);
+            int end = indices.get(indices.size() - 1);
+
+            if (end - start <= 1) continue;
+
+            HashSet<Character> seen = new HashSet<>();
+            for (int i = start + 1; i < end; i++) {
+                seen.add(s.charAt(i));
+            }
+            ans += seen.size();
+        }
+
+        return ans;
+    }
+}