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

Author: Gonzalo Diaz

src/algorithm_exercises_csharp/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/CountTriplets.cs

@@ -0,0 +1,42 @@
+// @link Problem definition [[docs/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/ctci-ransom-note.md]]
+
+namespace algorithm_exercises_csharp.hackerrank.interview_preparation_kit.dictionaries_and_hashmaps;
+
+using System.Collections.Generic;
+
+public class CountTriplets
+{
+  public static long countTriplets(List<long> arr, long r)
+  {
+    Dictionary<long, long> aCounter = [];
+    Dictionary<long, long> bCounter = [];
+    long triplets = 0L;
+
+    foreach (long item in arr)
+    {
+      aCounter[item] = aCounter.TryGetValue(item, out long value) ? value + 1L : 1L;
+    }
+
+    long prevItemCount;
+    long nextItemCount;
+
+    foreach (long item in arr)
+    {
+      long j = item / r;
+      long k = item * r;
+
+      aCounter[item] = aCounter[item] - 1L;
+
+      prevItemCount = bCounter.TryGetValue(j, out long bItem) ? bItem : 0L;
+      nextItemCount = aCounter.TryGetValue(k, out long aItem) ? aItem : 0L;
+      if (item % r == 0)
+      {
+        triplets += prevItemCount * nextItemCount;
+      }
+
+      bCounter[item] = bCounter.TryGetValue(item, out long currentItemCount) ? currentItemCount + 1L : 1L;
+    }
+
+    return triplets;
+  }
+}

src/algorithm_exercises_csharp/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/CountTripletsBruteForce.cs

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+// @link Problem definition [[docs/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/ctci-ransom-note.md]]
+
+namespace algorithm_exercises_csharp.hackerrank.interview_preparation_kit.dictionaries_and_hashmaps;
+
+using System.Diagnostics.CodeAnalysis;
+using System.Collections.Generic;
+
+public class CountTripletsBruteForce
+{
+  public static long countTriplets(List<long> arr, long r)
+  {
+    long size = arr.Count;
+    long counter = 0L;
+
+    for (int i = 0; i < size - 2; i++)
+    {
+      for (int j = i + 1; j < size - 1; j++)
+      {
+        for (int k = j + 1; k < size; k++)
+        {
+
+          if (r * arr[i] == arr[j] && r * arr[j] == arr[k])
+          {
+            counter += 1;
+          }
+        }
+      }
+    }
+
+    return counter;
+  }
+}

src/algorithm_exercises_csharp_test/Resources/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/count_triplets_1.big.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
+  }
+]

src/algorithm_exercises_csharp_test/Resources/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/count_triplets_1.small.testcases.json

@@ -0,0 +1,48 @@
+namespace algorithm_exercises_csharp_test.hackerrank.interview_preparation_kit.dictionaries_and_hashmaps;
+using algorithm_exercises_csharp_test.lib;
+using algorithm_exercises_csharp.hackerrank.interview_preparation_kit.dictionaries_and_hashmaps;
+
+[TestClass]
+public class CountTripletsTest
+{
+  public class CountTripletsTestCase
+  {
+    public string title { get; set; } = default!;
+    public List<long> input { get; set; } = default!;
+    public int r { get; set; } = default!;
+    public long expected { get; set; } = default!;
+  }
+
+  private List<CountTripletsTestCase> testCases { get; set; } = default!;
+  private List<CountTripletsTestCase> bigTestCases { get; set; } = default!;
+
+  [TestInitialize]
+  public void testInitialize()
+  {
+    testCases = JsonLoader.resourceLoad<List<CountTripletsTestCase>>(
+      "hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/count_triplets_1.small.testcases.json"
+    ) ?? [];
+
+    bigTestCases = JsonLoader.resourceLoad<List<CountTripletsTestCase>>(
+      "hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/count_triplets_1.big.testcases.json"
+    ) ?? [];
+  }
+
+  [TestMethod]
+  public void testCountTriplets()
+  {
+    long result;
+
+    foreach (CountTripletsTestCase test in testCases)
+    {
+      result = CountTriplets.countTriplets(test.input, test.r);
+      Assert.AreEqual(test.expected, result);
+    }
+
+    foreach (CountTripletsTestCase test in bigTestCases)
+    {
+      result = CountTriplets.countTriplets(test.input, test.r);
+      Assert.AreEqual(test.expected, result);
+    }
+  }
+}

src/algorithm_exercises_csharp_test/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/CountTriplets.Test.cs

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+namespace algorithm_exercises_csharp_test.hackerrank.interview_preparation_kit.dictionaries_and_hashmaps;
+using algorithm_exercises_csharp_test.lib;
+using algorithm_exercises_csharp.hackerrank.interview_preparation_kit.dictionaries_and_hashmaps;
+
+[TestClass]
+public class CountTripletsBruteForceTest
+{
+  public class CountTripletsBruteForceTestCase
+  {
+    public string title { get; set; } = default!;
+    public List<long> input { get; set; } = default!;
+    public int r { get; set; } = default!;
+    public int expected { get; set; } = default!;
+  }
+
+  private List<CountTripletsBruteForceTestCase> testCases { get; set; } = default!;
+
+  [TestInitialize]
+  public void testInitialize()
+  {
+    testCases = JsonLoader.resourceLoad<List<CountTripletsBruteForceTestCase>>(
+      "hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/count_triplets_1.small.testcases.json"
+    ) ?? [];
+  }
+
+  [TestMethod]
+  public void testCountTriplets()
+  {
+    long result;
+
+    foreach (CountTripletsBruteForceTestCase test in testCases)
+    {
+      result = CountTripletsBruteForce.countTriplets(test.input, test.r);
+      Assert.AreEqual(test.expected, result);
+    }
+  }
+}