Merge pull request #1995 from gnohgnij/1930-unique-3-length-palindromic-subsequences

Create: 1930-unique-3-length-palindromic-subsequences.java

Author: tahsintunan

Diff for: java/1930-unique-3-length-palindromic-subsequences.java

@@ -0,0 +1,32 @@
+class Solution {
+    public int countPalindromicSubsequence(String s) {
+        char[] letters = new char[]{'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k',
+                                    'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 
+                                    'w', 'x', 'y', 'z'};
+        Set<Character> left = new HashSet<>();
+        Map<Character, Integer> right = new HashMap<>();
+        for(char c : s.toCharArray()) {
+            right.put(c, right.getOrDefault(c, 0) + 1);
+        }
+        Set<String> res = new HashSet<>();
+
+        for(int mid = 0; mid < s.length(); mid++) {
+            char c = s.charAt(mid);
+            
+            right.put(c, right.get(c)-1);
+            if(right.get(c) == 0) {
+                right.remove(c);
+            }
+
+            for(int i=0; i<26; i++) {
+                if(left.contains(letters[i]) && right.containsKey(letters[i])) {
+                    res.add("" + letters[i] + c + letters[i]);
+                }
+            }
+
+            left.add(c);
+        }
+
+        return res.size();
+    }
+}