Added Java solution for "First Letter to Appear Twice"

Java/First Letter to Appear Twice.java

@@ -0,0 +1,41 @@
+import java.util.HashSet;
+import java.util.Set;
+
+public class Solution {
+
+    /**
+     * Time:   O(n)
+     * Memory: O(1)
+     */
+    public char repeatedCharacter(String s) {
+        Set<Character> seen = new HashSet<>();
+
+        for (char c : s.toCharArray()) {
+            if (seen.contains(c))
+                return c;
+            seen.add(c);
+        }
+
+        return '#';
+    }
+}
+
+
+public class Solution {
+
+    /**
+     * Time:   O(n)
+     * Memory: O(1)
+     */
+    public char repeatedCharacter(String s) {
+        boolean[] seen = new boolean[26];
+
+        for (char c : s.toCharArray()) {
+            if (seen[c - 'a'])
+                return c;
+            seen[c - 'a'] = true;
+        }
+
+        return '#';
+    }
+}