Runtime: 26 ms (Top 93.60%) | Memory: 51.8 MB (Top 51.74%)

Author: AnasImloul

scripts/algorithms/U/Unique Length3 Palindromic Subsequences/Unique Length-3 Palindromic Subsequences.java

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+// Runtime: 26 ms (Top 93.60%) | Memory: 51.8 MB (Top 51.74%)
+class Solution {
+    public int countPalindromicSubsequence(String s) {
+
+        int n = s.length();
+
+        char[] chArr = s.toCharArray();
+
+        int[] firstOcc = new int[26];
+        int[] lastOcc = new int[26];
+
+        Arrays.fill(firstOcc, -1);
+        Arrays.fill(lastOcc, -1);
+
+        for(int i = 0; i < n; i++){
+
+            char ch = chArr[i];
+
+            if(firstOcc[ch - 'a'] == -1){
+                firstOcc[ch - 'a'] = i;
+            }
+
+            lastOcc[ch - 'a'] = i;
+        }
+
+        int ans = 0, count = 0;
+
+        boolean[] visited;
+
+        // check for each character ( start or end of palindrome )
+        for(int i = 0; i < 26; i++){
+
+            int si = firstOcc[i]; // si - starting index
+            int ei = lastOcc[i]; // ei - ending index
+
+            visited = new boolean[26];
+
+            count = 0;
+
+            // check for unique charcters ( middle of palindrome )
+            for(int j = si + 1; j < ei; j++){
+
+                if(!visited[chArr[j] - 'a']){
+                    visited[chArr[j] - 'a'] = true;
+                    count++;
+                }
+            }
+
+            ans += count;
+        }
+
+        return ans;
+    }
+}