Implement UniqueEmailAddresses929 with complexity analysis and tests

src/main/java/com/leetcode/easy/UniqueEmailAddresses929.java

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+// Tags: Array, String, Hash Set
+package com.leetcode.easy;
+
+import java.util.HashSet;
+import java.util.Set;
+
+public class UniqueEmailAddresses929 {
+
+    /**
+     * Returns the number of unique email addresses after applying normalization rules.
+     * <p>
+     * Normalization rules:
+     * <ul>
+     *   <li>Dots (.) in the local name are ignored</li>
+     *   <li>Everything after the first plus (+) in the local name is ignored</li>
+     * </ul>
+     * <p>
+     * <b>Time Complexity: O(N × L)</b>
+     * <ul>
+     *   <li>N = number of emails in the input array</li>
+     *   <li>L = average length of an email address</li>
+     *   <li>For each email, we perform:
+     *     <ul>
+     *       <li>split("@") - O(L) to scan and split the string</li>
+     *       <li>replace(".", "") - O(L) to create new string without dots</li>
+     *       <li>split("\\+") - O(L) to find and split at '+' character</li>
+     *       <li>HashSet.add() - O(L) for hashing the string and O(1) average insertion</li>
+     *     </ul>
+     *   </li>
+     *   <li>Total: O(N × L) since we process each character of each email a constant number of times</li>
+     * </ul>
+     * <p>
+     * <b>Space Complexity: O(N × L)</b>
+     * <ul>
+     *   <li>HashSet stores up to N unique normalized emails</li>
+     *   <li>Each normalized email has length O(L) in the worst case</li>
+     *   <li>Additional temporary strings (split arrays, replaced strings) are O(L) each and get garbage collected</li>
+     *   <li>Worst case: all N emails are unique → O(N × L) total storage</li>
+     *   <li>Best case: all N emails normalize to the same address → O(L) storage</li>
+     * </ul>
+     *
+     * @param emails array of email addresses to process
+     * @return the count of unique email addresses
+     */
+    public int numUniqueEmails(String[] emails) {
+        Set<String> emailHashSet = new HashSet<>();
+        for (String email : emails) {
+            String[] parts = email.split("@");
+            String localName = parts[0].replace(".", "").split("\\+")[0];
+            emailHashSet.add(localName + "@" + parts[1]);
+        }
+        return emailHashSet.size();
+    }
+}

src/test/java/com/leetcode/easy/UniqueEmailAddresses929Test.java

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+package com.leetcode.easy;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class UniqueEmailAddresses929Test {
+    private UniqueEmailAddresses929 solution;
+
+    @BeforeEach
+    void setUp() {
+        solution = new UniqueEmailAddresses929();
+    }
+
+    @Test
+    void testExample1() {
+        String[] emails = {
+                "test.email+alex@leetcode.com",
+                "test.e.mail+bob.cathy@leetcode.com",
+                "testemail+david@lee.tcode.com"
+        };
+        assertEquals(2, solution.numUniqueEmails(emails));
+    }
+
+    @Test
+    void testExample2() {
+        String[] emails = {
+                "a@leetcode.com",
+                "b@leetcode.com",
+                "c@leetcode.com"
+        };
+        assertEquals(3, solution.numUniqueEmails(emails));
+    }
+}