solves #3120: Count the Number of Special Characters I in java

Author: anishLearnsToCode

README.md

@@ -905,7 +905,7 @@
 | 3105 | [Longest Strictly Increasing or Strictly Decreasing Subarray](https://leetcode.com/problems/longest-strictly-increasing-or-strictly-decreasing-subarray)               |                                                                    [![Java](assets/java.png)](src/LongestStrictlyIncreasingOrStrictlyDecreasingSubarray.java)                                                                    |                                                                                                                                   |
 | 3110 | [Score of a String](https://leetcode.com/problems/score-of-a-string)                                                                                                   |                                                                                       [![Java](assets/java.png)](src/ScoreOfAString.java)                                                                                        |                                                                                                                                   |
 | 3114 | [Latest Time You Can Obtain After Replacing Characters](https://leetcode.com/problems/latest-time-you-can-obtain-after-replacing-characters)                           |                                                                       [![Java](assets/java.png)](src/LatestTimeYouCanObtainAfterReplacingCharacters.java)                                                                        |                                                                                                                                   |
-| 3120 | [Count the Number of Special Characters I](https://leetcode.com/problems/count-the-number-of-special-characters-i)                                                     |                                                                                                                                                                                                                                  |                                                                                                                                   |
+| 3120 | [Count the Number of Special Characters I](https://leetcode.com/problems/count-the-number-of-special-characters-i)                                                     |                                                                             [![Java](assets/java.png)](src/CountTheNumberOfSpecialCharactersI.java)                                                                              |                                                                                                                                   |
 | 3127 | [Make a Square with the Same Color](https://leetcode.com/problems/make-a-square-with-the-same-color)                                                                   |                                                                                                                                                                                                                                  |                                                                                                                                   |
 | 3131 | [Find the Integer Added to Array I](https://leetcode.com/problems/find-the-integer-added-to-array-i)                                                                   |                                                                                                                                                                                                                                  |                                                                                                                                   |
 | 3136 | [Valid Word](https://leetcode.com/problems/valid-word)                                                                                                                 |                                                                                                                                                                                                                                  |                                                                                                                                   |

src/CountTheNumberOfSpecialCharactersI.java

@@ -0,0 +1,32 @@
+// https://leetcode.com/problems/count-the-number-of-special-characters-i
+// T: O(N)
+// S: O(1)
+
+import java.util.HashSet;
+import java.util.Set;
+
+public class CountTheNumberOfSpecialCharactersI {
+    public int numberOfSpecialChars(String word) {
+        final Set<Character> characters = new HashSet<>();
+        final Set<Character> alreadyCounted = new HashSet<>();
+        int specialChars = 0;
+
+        for (int i = 0 ; i < word.length() ; i++) {
+            characters.add(word.charAt(i));
+            if (characters.contains(inverse(word.charAt(i))) && !alreadyCounted.contains(word.charAt(i))) {
+                specialChars++;
+                alreadyCounted.add(word.charAt(i));
+                alreadyCounted.add(inverse(word.charAt(i)));
+            }
+        }
+
+        return specialChars;
+    }
+
+    private static char inverse(char c) {
+        if (Character.isLowerCase(c)) {
+            return Character.toUpperCase(c);
+        }
+        return Character.toLowerCase(c);
+    }
+}