[Hacker Rank] Interview Preparation Kit: Dictionaries and Hashmaps: Count Triplets. Solved ✓.

Author: Gonzalo Diaz

algorithm-exercises-java/src/main/java/ae/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/CountTriplets.java

@@ -0,0 +1,57 @@
+package ae.hackerrank.interview_preparation_kit.dictionaries_and_hashmaps;
+
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * CountTriplets.
+ *
+ * @link Problem definition
+ *       [[docs/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/two-strings.md]]
+ * @link Solution notes
+ *       [[docs/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/two-strings-solution-notes.md]]
+ */
+public class CountTriplets {
+  private CountTriplets() {
+  }
+
+  /**
+   * CountTriplets.
+   */
+  static long countTriplets(List<Long> arr, long r) {
+
+    Map<Long, Long> aCounter = new HashMap<>();
+    Map<Long, Long> bCounter = new HashMap<>();
+    long triplets = 0L;
+
+    for (Long item : arr) {
+      if (aCounter.putIfAbsent(item, 1L) != null) {
+        Long currentValue = aCounter.get(item);
+        aCounter.put(item, currentValue + 1L);
+      }
+    }
+
+    long jItem;
+    long kItem;
+
+    for (Long item : arr) {
+      Long j = item / r;
+      Long k = item * r;
+
+      aCounter.put(item, aCounter.get(item) - 1L);
+
+      jItem = bCounter.get(j) != null ? bCounter.get(j) : 0L;
+      kItem = aCounter.get(k) != null ? aCounter.get(k) : 0L;
+      if (item % r == 0) {
+        triplets += jItem * kItem;
+      }
+
+      Long bItem = bCounter.get(item);
+
+      bCounter.put(item, bItem != null ? bItem + 1L : 1L);
+    }
+
+    return triplets;
+  }
+}

algorithm-exercises-java/src/test/java/ae/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/CountTripletsTest.java

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+package ae.hackerrank.interview_preparation_kit.dictionaries_and_hashmaps;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.io.IOException;
+import java.util.List;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.TestInstance;
+import org.junit.jupiter.api.TestInstance.Lifecycle;
+import util.JsonLoader;
+
+/**
+ * CountTripletsTest.
+ */
+@TestInstance(Lifecycle.PER_CLASS)
+class CountTripletsTest {
+  public static class CountTripletsTestCase {
+    public String title;
+    public List<Long> input;
+    public Long r;
+    public Long expected;
+  }
+
+  private List<CountTripletsTestCase> smallTestCases;
+  private List<CountTripletsTestCase> bigTestCases;
+  private List<CountTripletsTestCase> borderTestCases;
+
+  @BeforeAll
+  void setup() throws IOException {
+    String path;
+    path = String.join("/",
+        "hackerrank",
+        "interview_preparation_kit",
+        "dictionaries_and_hashmaps",
+        "count_triplets_1.small.testcases.json");
+
+    this.smallTestCases = JsonLoader.loadJson(path, CountTripletsTestCase.class);
+
+    path = String.join("/",
+        "hackerrank",
+        "interview_preparation_kit",
+        "dictionaries_and_hashmaps",
+        "count_triplets_1.big.testcases.json");
+
+    this.bigTestCases = JsonLoader.loadJson(path, CountTripletsTestCase.class);
+
+    path = String.join("/",
+        "hackerrank",
+        "interview_preparation_kit",
+        "dictionaries_and_hashmaps",
+        "count_triplets_1.border.testcases.json");
+
+    this.borderTestCases = JsonLoader.loadJson(path, CountTripletsTestCase.class);
+
+  }
+
+  @Test
+  void testCountTriplets() {
+    for (CountTripletsTestCase test : smallTestCases) {
+      Long solutionFound = CountTriplets.countTriplets(test.input, test.r);
+
+      assertEquals(test.expected, solutionFound,
+          "%s(%s, %d) answer must be: %d".formatted(
+              "CountTriplets.countTriplets",
+              test.input.toString(),
+              test.r,
+              test.expected));
+    }
+  }
+
+  @Test
+  void testCountTripletsBigCases() {
+    for (CountTripletsTestCase test : bigTestCases) {
+      Long solutionFound = CountTriplets.countTriplets(test.input, test.r);
+
+      assertEquals(test.expected, solutionFound,
+          "%s(%s, %d) answer must be: %d".formatted(
+              "CountTriplets.countTriplets",
+              test.input.toString(),
+              test.r,
+              test.expected));
+    }
+  }
+}

algorithm-exercises-java/src/test/resources/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/count_triplets_1.big.testcases.json

@@ -0,0 +1,8 @@
+[
+  {
+    "title": "Border test case 0",
+    "input": [1, 2, 3, 4],
+    "r": 2,
+    "expected": 1
+  }
+]

algorithm-exercises-java/src/test/resources/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/count_triplets_1.border.testcases.json

@@ -0,0 +1,26 @@
+[
+  {
+    "title": "Sample Test Case 0",
+    "input": [1, 2, 2, 4],
+    "r": 2,
+    "expected": 2
+  },
+  {
+    "title": "Sample Test Case 1",
+    "input": [1, 3, 9, 9, 27, 81],
+    "r": 3,
+    "expected": 6
+  },
+  {
+    "title": "Sample Test Case 1 (unsorted)",
+    "input": [9, 3, 1, 81, 9, 27],
+    "r": 3,
+    "expected": 1
+  },
+  {
+    "title": "Sample Test Case 12",
+    "input": [1, 5, 5, 25, 125],
+    "r": 5,
+    "expected": 4
+  }
+]