solves #2351: First Letter to Appear Twice in java

Author: anishLearnsToCode

README.md

@@ -754,7 +754,7 @@
 | 2341 | [Maximum Number of Pairs in Array](https://leetcode.com/problems/maximum-number-of-pairs-in-array)                                                                     |                                                                                 [![Java](assets/java.png)](src/MaximumNumberOfPairsInArray.java)                                                                                 |                                                                                                                                   |
 | 2347 | [Best Poker Hand](https://leetcode.com/problems/best-poker-hand)                                                                                                       |                                                                                        [![Java](assets/java.png)](src/BestPokerHand.java)                                                                                        |                                                                                                                                   |
 | 2348 | [Number of Zero-Filled Subarrays](https://leetcode.com/problems/number-of-zero-filled-subarrays)                                                                       |                                                                            [![Python](assets/python.png)](python/number_of_zero_filled_subarrays.py)                                                                             |                                                                                                                                   |
-| 2351 | [First Letter to Appear Twice](https://leetcode.com/problems/first-letter-to-appear-twice)                                                                             |                                                                                                                                                                                                                                  |                                                                                                                                   |
+| 2351 | [First Letter to Appear Twice](https://leetcode.com/problems/first-letter-to-appear-twice)                                                                             |                                                                                  [![Java](assets/java.png)](src/FirstLetterToAppearTwice.java)                                                                                   |                                                                                                                                   |
 | 2357 | [Make Array Zero by Subtracting Equal Amounts](https://leetcode.com/problems/make-array-zero-by-subtracting-equal-amounts)                                             |                                                                                                                                                                                                                                  |                                                                                                                                   |
 | 2360 | [Longest Cycle in a Graph](https://leetcode.com/problems/longest-cycle-in-a-graph)                                                                                     |                                                                                 [![Python](assets/python.png)](python/longest_cycle_in_graph.py)                                                                                 |                                                                                                                                   |
 | 2363 | [Merge Similar Items](https://leetcode.com/problems/merge-similar-items)                                                                                               |                                                                                                                                                                                                                                  |                                                                                                                                   |

src/FirstLetterToAppearTwice.java

@@ -0,0 +1,19 @@
+// https://leetcode.com/problems/first-letter-to-appear-twice
+// T: O(1)
+// S: O(1)
+
+import java.util.HashSet;
+import java.util.Set;
+
+public class FirstLetterToAppearTwice {
+    public char repeatedCharacter(String s) {
+        final Set<Character> characters = new HashSet<>();
+        for (int index = 0 ; index < s.length() ; index++) {
+            if (characters.contains(s.charAt(index))) {
+                return s.charAt(index);
+            }
+            characters.add(s.charAt(index));
+        }
+        return 'a';
+    }
+}