problem(442):find-all-duplicates-in-an-array

src/main/java/problems/FindAllDuplicatesInAnArray.java

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+package problems;
+
+import lombok.AccessLevel;
+import lombok.NoArgsConstructor;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+
+/**
+ * <a href="https://leetcode.com/problems/find-all-duplicates-in-an-array">find-all-duplicates-in-an-array</a>
+ */
+@NoArgsConstructor(access = AccessLevel.PRIVATE)
+public class FindAllDuplicatesInAnArray {
+    static List<Integer> findDuplicates(int[] nums) {
+        if (nums == null || nums.length == 0) {
+            return Collections.emptyList();
+        }
+        int max = Arrays.stream(nums).max().getAsInt();
+        int[] count = new int[max + 1];
+        for (int num : nums) {
+            count[num]++;
+        }
+        List<Integer> duplicates = new ArrayList<>();
+        for (int i = 0; i < count.length; i++) {
+            if (count[i] > 1) {
+                duplicates.add(i);
+            }
+        }
+        return duplicates;
+    }
+
+
+}

src/test/java/problems/FindAllDuplicatesInAnArrayTest.java

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+package problems;
+
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+import java.util.List;
+import java.util.stream.Stream;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class FindAllDuplicatesInAnArrayTest {
+
+    public static Stream<Arguments> numbers() {
+        return Stream.of(
+                Arguments.of(new int[]{4,3,2,7,8,2,3,1}, List.of(2,3)),
+                Arguments.of(new int[]{1,1,2}, List.of(1)),
+                Arguments.of(new int[]{1}, List.of()),
+                Arguments.of(new int[]{}, List.of()),
+                Arguments.of(null, List.of())
+        );
+    }
+
+    @ParameterizedTest
+    @MethodSource("numbers")
+    void findDuplicates(int[] arr, List<Integer> expected) {
+        assertThat(FindAllDuplicatesInAnArray.findDuplicates(arr)).isEqualTo(expected);
+    }
+}