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[Hacker Rank] Interview Preparation Kit: Dictionaries and Hashmaps: C... #193

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# [Dictionaries and Hashmaps: Count Triplets](https://www.hackerrank.com/challenges/count-triplets-1)

- Difficulty: `#medium`
- Category: `#ProblemSolvingIntermediate`

## Working solution

This solution in O(N), is based on considering that each
number analyzed is in the center of the triplet and asks
"how many" possible cases there are on the left and
right to calculate how many possible triplets are formed.

- 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)

- Difficulty: `#medium`
- Category: `#ProblemSolvingIntermediate`

You are given an array and you need to find number of
tripets of indices (i, j, k) such that the elements at
those indices are in geometric progression for a given
common ratio `r` and $ i < j < k $.

## Example

`arr = [1, 4, 16, 64] r = 4`

There are `[1, 4, 16]` and `[4, 16, 64]` at indices (0, 1, 2) and (1, 2, 3).
Return `2`.

## Function Description

Complete the countTriplets function in the editor below.

countTriplets has the following parameter(s):

- `int arr[n]`: an array of integers
- `int r`: the common ratio

## Returns

- `int`: the number of triplets

## Input Format

The first line contains two space-separated integers `n` and `r`,
the size of `arr` and the common ratio.
The next line contains `n` space-seperated integers `arr[i]`.

## Constraints

- $ 1 \leq n \leq 10^5 $
- $ 1 \leq r \leq 10^9 $
- $ 1 \leq arr[i] \leq 10^9 $

## Sample Input 0

```text
4 2
1 2 2 4
```

## Sample Output 0

```text
2
```

## Explanation 0

There are `2` triplets in satisfying our criteria,
whose indices are (0, 1, 3) and (0, 2, 3)

## Sample Input 1

```text
6 3
1 3 9 9 27 81
```

## Sample Output 1

```text
6
```

## Explanation 1

The triplets satisfying are index
`(0, 1, 2)`, `(0, 1, 3)`, `(1, 2, 4)`, `(1, 3, 4)`, `(2, 4, 5)` and `(3, 4, 5)`.

## Sample Input 2

```text
5 5
1 5 5 25 125
```

## Sample Output 2

```text
4
```

## Explanation 2

The triplets satisfying are index
`(0, 1, 3)`, `(0, 2, 3)`, `(1, 2, 3)`, `(2, 3, 4)`.

## Appendix

[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]]

// @link Solution notes [[docs/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/count_triplets_1-solution-notes.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;
}
}
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// @link Problem definition [[docs/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/count_triplets_1.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;
}
}

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[
{
"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
}
]
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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 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);
}
}
}
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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);
}
}
}

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