Add solution for "The Two Sneaky Numbers of Digitville" with unit tests.

src/main/java/com/leetcode/easy/TheTwoSneakyNumbersOfDigitville3289.java

@@ -0,0 +1,34 @@
+// Tags: Array, Hash Set
+package com.leetcode.easy;
+
+import java.util.HashSet;
+import java.util.Set;
+
+public class TheTwoSneakyNumbersOfDigitville3289 {
+    /**
+     * Finds the two numbers that appear twice in the array.
+     * <p>
+     * Time Complexity: O(n) where n is the length of the array.
+     * We iterate through the array once, and HashSet operations (add, contains) are O(1) on average.
+     * <p>
+     * Space Complexity: O(n) for the HashSet to store seen numbers.
+     * In the worst case, we store almost all numbers before finding the duplicates.
+     */
+    public int[] getSneakyNumbers(int[] nums) {
+        int[] res = new int[2];
+        Set<Integer> seen = new HashSet<>();
+        int index = 0;
+        for (int num : nums) {
+            if (seen.contains(num)) {
+                res[index] = num;
+                if (index == 1) {
+                    break;
+                }
+                index++;
+            } else {
+                seen.add(num);
+            }
+        }
+        return res;
+    }
+}

src/test/java/com/leetcode/easy/TheTwoSneakyNumbersOfDigitville3289Test.java

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+package com.leetcode.easy;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.util.Arrays;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+
+public class TheTwoSneakyNumbersOfDigitville3289Test {
+    private TheTwoSneakyNumbersOfDigitville3289 solution;
+
+    @BeforeEach
+    void setUp() {
+        solution = new TheTwoSneakyNumbersOfDigitville3289();
+    }
+
+    @Test
+    void testGetSneakyNumbers_Example1() {
+        int[] nums = {0, 1, 1, 0};
+        int[] expected = {0, 1};
+
+        int[] result = solution.getSneakyNumbers(nums);
+        Arrays.sort(result);
+
+        assertArrayEquals(expected, result);
+    }
+
+    @Test
+    void testGetSneakyNumbers_Example2() {
+        int[] nums = {0, 3, 2, 1, 3, 2};
+        int[] expected = {2, 3};
+
+        int[] result = solution.getSneakyNumbers(nums);
+        Arrays.sort(result);
+
+        assertArrayEquals(expected, result);
+    }
+
+    @Test
+    void testGetSneakyNumbers_Example3() {
+        int[] nums = {7, 1, 5, 4, 3, 4, 6, 0, 9, 5, 8, 2};
+        int[] expected = {4, 5};
+
+        int[] result = solution.getSneakyNumbers(nums);
+        Arrays.sort(result);
+
+        assertArrayEquals(expected, result);
+    }
+}