solves #2309: Greatest English Letter in Upper and Lower Case in java

Author: anishLearnsToCode

Diff for: README.md

@@ -742,7 +742,8 @@
 | 2293 | [Min Max Game](https://leetcode.com/problems/min-max-game)                                                                                                             |                                                                                         [![Java](assets/java.png)](src/MinMaxGame.java)                                                                                          |                                                                                                                                   |
 | 2299 | [Strong Password Checker II](https://leetcode.com/problems/strong-password-checker-ii)                                                                                 |                                                                                   [![Java](assets/java.png)](src/StrongPasswordCheckerII.java)                                                                                   |                                                                                                                                   |
 | 2300 | [Successful Pairs of Spells and Potions](https://leetcode.com/problems/successful-pairs-of-spells-and-potions)                                                         |                                                                        [![Python](assets/python.png)](python/successfull_pairs_of_spells_and_potions.py)                                                                         |                                                                                                                                   |
-| 2303 | [Calculate Amount Paid in Taxes](https://leetcode.com/problems/calculate-amount-paid-in-taxes)                                                                         |                                        [![Java](assets/java.png)](src/CalculateAmountPaidInTaxes.java) [![Python](assets/python.png)](python/successfull_pairs_of_spells_and_potions.py)                                         |                                                                                                                                   |
+| 2303 | [Calculate Amount Paid in Taxes](https://leetcode.com/problems/calculate-amount-paid-in-taxes)                                                                         |                                                                                 [![Java](assets/java.png)](src/CalculateAmountPaidInTaxes.java)                                                                                  |                                                                                                                                   |
+| 2309 | [Greatest English Letter in Upper and Lower Case](https://leetcode.com/problems/greatest-english-letter-in-upper-and-lower-case)                                       |                                                                          [![Java](assets/java.png)](src/GreatestEnglishLetterInUpperAndLowerCase.java)                                                                           |                                                                                                                                   |
 | 2315 | [Count Asterisks](https://leetcode.com/problems/count-asterisks)                                                                                                       |                                                                        [![Python](assets/python.png)](python/successfull_pairs_of_spells_and_potions.py)                                                                         |                                                                                                                                   |
 | 2316 | [Count Unreachable Pairs of Nodes in an Undirected Graph](https://leetcode.com/problems/count-unreachable-pairs-of-nodes-in-an-undirected-graph)                       |                                                                [![Python](assets/python.png)](python/count_unreachable_pair_of_node_in_an_undirectable_graph.py)                                                                 |                                                                                                                                   |
 | 2319 | [Check if Matrix Is X-Matrix](https://leetcode.com/problems/check-if-matrix-is-x-matrix)                                                                               |                                                                [![Python](assets/python.png)](python/count_unreachable_pair_of_node_in_an_undirectable_graph.py)                                                                 |                                                                                                                                   |

Diff for: src/GreatestEnglishLetterInUpperAndLowerCase.java

@@ -0,0 +1,42 @@
+// https://leetcode.com/problems/greatest-english-letter-in-upper-and-lower-case
+// T: O(|s|)
+// S: O(1)
+
+import java.util.HashSet;
+import java.util.Set;
+
+public class GreatestEnglishLetterInUpperAndLowerCase {
+    public String greatestLetter(String s) {
+        final Set<Character> lowercaseLetters = getLowercaseLetters(s);
+        final Set<Character> uppercaseLetters = getUppercaseLetters(s);
+        char maxLetter = 'a' - 1;
+
+        for (char letter : lowercaseLetters) {
+            if (uppercaseLetters.contains(Character.toUpperCase(letter)) && letter > maxLetter) {
+                maxLetter = letter;
+            }
+        }
+
+        return maxLetter >= 'a' ? (Character.toUpperCase(maxLetter) + "") : "";
+    }
+
+    private Set<Character> getUppercaseLetters(String string) {
+        final Set<Character> set = new HashSet<>();
+        for (int i = 0 ; i < string.length() ; i++) {
+            if (Character.isUpperCase(string.charAt(i))) {
+                set.add(string.charAt(i));
+            }
+        }
+        return set;
+    }
+
+    private Set<Character> getLowercaseLetters(String string) {
+        final Set<Character> set = new HashSet<>();
+        for (int i = 0 ; i < string.length() ; i++) {
+            if (Character.isLowerCase(string.charAt(i))) {
+                set.add(string.charAt(i));
+            }
+        }
+        return set;
+    }
+}