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Commit 0cc5b42

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Gonzalo Diaz
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[Hacker Rank] Interview Preparation Kit: Dictionaries and Hashmaps: Count Triplets. Solved ✓.
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# [Dictionaries and Hashmaps: Count Triplets](https://www.hackerrank.com/challenges/count-triplets-1)
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- Difficulty: `#medium`
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- Category: `#ProblemSolvingIntermediate`
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## Working solution
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This solution in O(N), is based on considering that each
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number analyzed is in the center of the triplet and asks
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"how many" possible cases there are on the left and
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right to calculate how many possible triplets are formed.
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- Source: [Hackerrank - Count Triplets Solution](https://www.thepoorcoder.com/hackerrank-count-triplets-solution/)
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# [Dictionaries and Hashmaps: Count Triplets](https://www.hackerrank.com/challenges/count-triplets-1)
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- Difficulty: `#medium`
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- Category: `#ProblemSolvingIntermediate`
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You are given an array and you need to find number of
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tripets of indices (i, j, k) such that the elements at
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those indices are in geometric progression for a given
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common ratio `r` and $ i < j < k $.
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## Example
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`arr = [1, 4, 16, 64] r = 4`
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There are `[1, 4, 16]` and `[4, 16, 64]` at indices (0, 1, 2) and (1, 2, 3).
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Return `2`.
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## Function Description
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Complete the countTriplets function in the editor below.
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countTriplets has the following parameter(s):
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- `int arr[n]`: an array of integers
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- `int r`: the common ratio
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## Returns
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- `int`: the number of triplets
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## Input Format
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The first line contains two space-separated integers `n` and `r`,
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the size of `arr` and the common ratio.
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The next line contains `n` space-seperated integers `arr[i]`.
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## Constraints
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- $ 1 \leq n \leq 10^5 $
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- $ 1 \leq r \leq 10^9 $
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- $ 1 \leq arr[i] \leq 10^9 $
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## Sample Input 0
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```text
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4 2
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1 2 2 4
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```
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## Sample Output 0
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```text
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2
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```
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## Explanation 0
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There are `2` triplets in satisfying our criteria,
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whose indices are (0, 1, 3) and (0, 2, 3)
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## Sample Input 1
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```text
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6 3
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1 3 9 9 27 81
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```
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## Sample Output 1
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```text
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6
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```
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## Explanation 1
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The triplets satisfying are index
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`(0, 1, 2)`, `(0, 1, 3)`, `(1, 2, 4)`, `(1, 3, 4)`, `(2, 4, 5)` and `(3, 4, 5)`.
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## Sample Input 2
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```text
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5 5
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1 5 5 25 125
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```
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## Sample Output 2
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```text
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4
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```
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## Explanation 2
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The triplets satisfying are index
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`(0, 1, 3)`, `(0, 2, 3)`, `(1, 2, 3)`, `(2, 3, 4)`.
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## Appendix
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[Solution notes](count_triplets_1-solution-notes.md)
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// @link Problem definition [[docs/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/count_triplets_1.md]]
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// @link Solution notes [[docs/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/count_triplets_1-solution-notes.md]]
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namespace algorithm_exercises_csharp.hackerrank.interview_preparation_kit.dictionaries_and_hashmaps;
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using System.Collections.Generic;
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public class CountTriplets
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{
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public static long countTriplets(List<long> arr, long r)
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{
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Dictionary<long, long> aCounter = [];
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Dictionary<long, long> bCounter = [];
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long triplets = 0L;
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foreach (long item in arr)
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{
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aCounter[item] = aCounter.TryGetValue(item, out long value) ? value + 1L : 1L;
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}
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long prevItemCount;
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long nextItemCount;
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foreach (long item in arr)
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{
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long j = item / r;
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long k = item * r;
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aCounter[item] = aCounter[item] - 1L;
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prevItemCount = bCounter.TryGetValue(j, out long bItem) ? bItem : 0L;
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nextItemCount = aCounter.TryGetValue(k, out long aItem) ? aItem : 0L;
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if (item % r == 0)
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{
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triplets += prevItemCount * nextItemCount;
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}
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bCounter[item] = bCounter.TryGetValue(item, out long currentItemCount) ? currentItemCount + 1L : 1L;
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}
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return triplets;
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}
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}
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// @link Problem definition [[docs/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/count_triplets_1.md]]
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namespace algorithm_exercises_csharp.hackerrank.interview_preparation_kit.dictionaries_and_hashmaps;
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using System.Diagnostics.CodeAnalysis;
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using System.Collections.Generic;
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public class CountTripletsBruteForce
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{
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public static long countTriplets(List<long> arr, long r)
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{
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long size = arr.Count;
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long counter = 0L;
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for (int i = 0; i < size - 2; i++)
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{
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for (int j = i + 1; j < size - 1; j++)
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{
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for (int k = j + 1; k < size; k++)
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{
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if (r * arr[i] == arr[j] && r * arr[j] == arr[k])
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{
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counter += 1;
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}
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}
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}
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}
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return counter;
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}
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}

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

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[
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{
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"title": "Sample Test Case 0",
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"input": [1, 2, 2, 4],
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"r": 2,
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"expected": 2
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},
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{
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"title": "Sample Test Case 1",
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"input": [1, 3, 9, 9, 27, 81],
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"r": 3,
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"expected": 6
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},
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{
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"title": "Sample Test Case 1 (unsorted)",
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"input": [9, 3, 1, 81, 9, 27],
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"r": 3,
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"expected": 1
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},
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{
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"title": "Sample Test Case 12",
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"input": [1, 5, 5, 25, 125],
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"r": 5,
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"expected": 4
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}
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]
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namespace algorithm_exercises_csharp_test.hackerrank.interview_preparation_kit.dictionaries_and_hashmaps;
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using algorithm_exercises_csharp_test.lib;
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using algorithm_exercises_csharp.hackerrank.interview_preparation_kit.dictionaries_and_hashmaps;
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[TestClass]
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public class CountTripletsTest
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{
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public class CountTripletsTestCase
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{
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public string title { get; set; } = default!;
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public List<long> input { get; set; } = default!;
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public int r { get; set; } = default!;
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public long expected { get; set; } = default!;
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}
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private List<CountTripletsTestCase> testCases { get; set; } = default!;
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private List<CountTripletsTestCase> bigTestCases { get; set; } = default!;
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[TestInitialize]
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public void testInitialize()
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{
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testCases = JsonLoader.resourceLoad<List<CountTripletsTestCase>>(
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"hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/count_triplets_1.small.testcases.json"
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) ?? [];
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bigTestCases = JsonLoader.resourceLoad<List<CountTripletsTestCase>>(
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"hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/count_triplets_1.big.testcases.json"
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) ?? [];
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}
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[TestMethod]
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public void testCountTriplets()
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{
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long result;
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foreach (CountTripletsTestCase test in testCases)
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{
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result = CountTriplets.countTriplets(test.input, test.r);
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Assert.AreEqual(test.expected, result);
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}
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foreach (CountTripletsTestCase test in bigTestCases)
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{
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result = CountTriplets.countTriplets(test.input, test.r);
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Assert.AreEqual(test.expected, result);
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}
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}
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}
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namespace algorithm_exercises_csharp_test.hackerrank.interview_preparation_kit.dictionaries_and_hashmaps;
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using algorithm_exercises_csharp_test.lib;
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using algorithm_exercises_csharp.hackerrank.interview_preparation_kit.dictionaries_and_hashmaps;
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[TestClass]
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public class CountTripletsBruteForceTest
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{
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public class CountTripletsBruteForceTestCase
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{
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public string title { get; set; } = default!;
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public List<long> input { get; set; } = default!;
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public int r { get; set; } = default!;
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public int expected { get; set; } = default!;
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}
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private List<CountTripletsBruteForceTestCase> testCases { get; set; } = default!;
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[TestInitialize]
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public void testInitialize()
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{
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testCases = JsonLoader.resourceLoad<List<CountTripletsBruteForceTestCase>>(
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"hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/count_triplets_1.small.testcases.json"
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) ?? [];
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}
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[TestMethod]
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public void testCountTriplets()
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{
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long result;
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foreach (CountTripletsBruteForceTestCase test in testCases)
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{
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result = CountTripletsBruteForce.countTriplets(test.input, test.r);
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Assert.AreEqual(test.expected, result);
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}
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}
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}

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