# Count triplets in an array such that i<j<k and a[j] – a[i] = a[k] – a[j] = D

Given an array arr and an integer D, the task is to count the number of triplets(arr[i], arr[j], arr[k]) in the array such that:

1. i < j < k
2. arr[j] – arr[i] = arr[k] – arr[j] = D

Examples:

Input: arr = {1, 6, 7, 7, 8, 10, 12, 13, 14}, D = 3
Output:
Explanation:
There are two triplets in the array that satisfies the given criteria.
-> 1st triplet(7, 10, 13) such that i = 3, j = 6 and k = 8, such that (i < j < k) and (10 – 7 = 13 – 10 = D (=3))
-> 2nd triplet(7, 10, 13) such that i = 4, j = 6 and k = 8, such that (i < j < k) and (10 – 7 = 13 – 10 = D (=3))

Input: arr = {6, 3, 4, 5}, D = 1
Output:

Naive Approach: The simplest approach is using three nested for loops and find the triplets that satisfy the given criteria. Below is the implementation of this approach.

## C++

 `// C++ program to count the triplets` `#include` `using` `namespace` `std;`   `// Function to count the triplets` `int` `CountTriplets(``int` `arr[], ``int` `d, ``int` `n)` `{` `    ``int` `count = 0;`   `    ``// Three nested for loops to count the` `    ``// triplets that satisfy the given criteria` `    ``for``(``int` `i = 0; i < n - 2; i++)` `    ``{` `       ``for``(``int` `j = i + 1; j < n - 1; j++)` `       ``{` `          ``for``(``int` `k = j + 1; k < n; k++)` `          ``{` `             ``if` `((arr[j] - arr[i] == d) && ` `                 ``(arr[k] - arr[j] == d)) ` `             ``{` `                 ``count++;` `             ``}` `          ``}` `       ``}` `    ``}` `    ``return` `count;` `}`   `// Driver Code` `int` `main()` `{` `    ``int` `A[] = { 1, 6, 7, 7, 8, 10, 12, 13, 14 };` `    ``int` `D = 3;` `    ``int` `n = ``sizeof``(A) / ``sizeof``(A[0]);` `    `  `    ``cout << (CountTriplets(A, D, n));` `}`   `// This code is contributed by chitranayal`

## Java

 `// Java program to count the triplets`   `class` `GFG {`   `    ``// Function to count the triplets` `    ``static` `int` `CountTriplets(``int``[] arr, ``int` `d)` `    ``{` `        ``int` `count = ``0``;` `        ``int` `n = arr.length;`   `        ``// Three nested for loops to count the` `        ``// triplets that satisfy the given criteria` `        ``for` `(``int` `i = ``0``; i < n - ``2``; i++) {` `            ``for` `(``int` `j = i + ``1``; j < n - ``1``; j++) {` `                ``for` `(``int` `k = j + ``1``; k < n; k++) {`   `                    ``if` `((arr[j] - arr[i] == d)` `                        ``&& (arr[k] - arr[j] == d)) {` `                        ``count++;` `                    ``}` `                ``}` `            ``}` `        ``}` `        ``return` `count;` `    ``}`   `    ``// Driver Code` `    ``public` `static` `void` `main(String args[])` `    ``{` `        ``int` `A[] = { ``1``, ``6``, ``7``, ``7``, ``8``, ``10``, ``12``, ``13``, ``14` `};` `        ``int` `D = ``3``;` `        ``System.out.println(CountTriplets(A, D));` `    ``}` `}`

## Python3

 `# Python3 program to count the triplets`   `# Function to count the triplets` `def` `CountTriplets(arr, d):` `    `  `    ``count ``=` `0``;` `    ``n ``=` `len``(arr);`   `    ``# Three nested for loops to count the` `    ``# triplets that satisfy the given criteria` `    ``for` `i ``in` `range``(n ``-` `1``):` `        ``for` `j ``in` `range``(i ``+` `1``, n ``-` `1``):` `            ``for` `k ``in` `range``(j ``+` `1``, n):`   `                ``if` `((arr[j] ``-` `arr[i] ``=``=` `d) ``and` `                    ``(arr[k] ``-` `arr[j] ``=``=` `d)):` `                    ``count ``+``=` `1``;` `    ``return` `count;`   `# Driver Code` `if` `__name__ ``=``=` `'__main__'``:` `    `  `    ``A ``=` `[ ``1``, ``6``, ``7``, ``7``, ``8``, ``10``, ``12``, ``13``, ``14` `];` `    ``D ``=` `3``;` `    `  `    ``print``(CountTriplets(A, D));`   `# This code is contributed by Rajput-Ji`

## C#

 `// C# program to count the triplets ` `using` `System;`   `class` `GFG {` ` `  `    ``// Function to count the triplets` `    ``static` `int` `CountTriplets(``int``[] arr, ``int` `d)` `    ``{` `        ``int` `count = 0;` `        ``int` `n = arr.Length;` ` `  `        ``// Three nested for loops to count the` `        ``// triplets that satisfy the given criteria` `        ``for` `(``int` `i = 0; i < n - 2; i++) {` `            ``for` `(``int` `j = i + 1; j < n - 1; j++) {` `                ``for` `(``int` `k = j + 1; k < n; k++) {` ` `  `                    ``if` `((arr[j] - arr[i] == d)` `                        ``&& (arr[k] - arr[j] == d)) {` `                        ``count++;` `                    ``}` `                ``}` `            ``}` `        ``}` `        ``return` `count;` `    ``}` ` `  `    ``// Driver Code` `    ``public` `static` `void` `Main(String []args)` `    ``{` `        ``int` `[]A = { 1, 6, 7, 7, 8, 10, 12, 13, 14 };` `        ``int` `D = 3;` `        ``Console.WriteLine(CountTriplets(A, D));` `    ``}` `}`   `// This code is contributed by PrinciRaj1992`

## Javascript

 ``

Output:

`2`

Time Complexity: O(N3).
Auxiliary Space: O(1), no extra space is required, so it is a constant.

Efficient Approach:

• An efficient approach for this problem is to use a map to store (key, values) pair where value will be count of key.
• The idea is to traverse the array from 0 to N and do following:
• check that A[i] – D and A[i] – 2*D are present in the map or not.
• If it’s in the map then we will simply multiply their respective values and increase answer by that.
• Now, we just have to put A[i] into the map and update the map.

Below is the implementation of the above approach.

## C++14

 `// C++14 program to count the number ` `// of triplets from an array.` `#include` `using` `namespace` `std; `   `// Function to count the triplets ` `int` `countTriplets (``int` `arr[], ``int` `d, ``int` `n)` `{` `    ``int` `count = -1;` `    `  `    ``// Create a map to store (key, values) pair. ` `    ``map<``int``, ``int``> set;`   `    ``// Traverse the array and check that we ` `    ``// have already put (a[i]-d and a[i]-2*d) ` `    ``// into map or not. If yes we have to get ` `    ``// the values of both the keys and ` `    ``// multiply them, else put a[i] into the map. ` `    ``for``(``int` `i = 0; i < n; i++)` `    ``{` `        ``if` `((set.find(arr[i] - d) != set.end()) &&` `            ``(set.find(arr[i] - 2 * d) != set.end()))` `        ``{` `            ``count += (set[arr[i] - d] * ` `                      ``set[arr[i] - 2 * d]);` `        ``}`   `        ``// Update the map` `        ``if` `(set.find(arr[i]) == set.end())` `            ``set[arr[i]] = 1;` `        ``else` `            ``set[arr[i]] += 1;` `    ``}` `    ``return` `count;` `}`   `// Driver Code` `int` `main() ` `{ ` `    `  `    ``// Test Case 1: ` `    ``int` `a1[] = { 1, 6, 7, 7, 8, 10, 12, 13, 14 }; ` `    ``int` `d1 = 3; ` `    ``cout << countTriplets(a1, d1, 9) << endl; `   `    ``// Test Case 2: ` `    ``int` `a2[] = { 6, 3, 4, 5 }; ` `    ``int` `d2 = 1; ` `    ``cout << countTriplets(a2, d2, 4) << endl; `   `    ``// Test Case 3: ` `    ``int` `a3[] = { 1, 2, 4, 5, 7, 8, 10 }; ` `    ``int` `d3 = 3; ` `    ``cout << countTriplets(a3, d3, 7) << endl; `   `    ``return` `0; ` `} `   `// This code is contributed by himanshu77`

## Java

 `// Java program to count the number` `// of triplets from an array.`   `import` `java.util.*;`   `class` `GFG {`   `    ``// Function to count the triplets` `    ``static` `int` `countTriplets(``int``[] arr, ``int` `d)` `    ``{` `        ``int` `count = -``1``;`   `        ``// Create a map to store (key, values) pair.` `        ``Map set = ``new` `HashMap<>();`   `        ``// Traverse the array and check that we` `        ``// have already put (a[i]-d and a[i]-2*d)` `        ``// into map or not. If yes we have to get` `        ``// the values of both the keys and` `        ``// multiply them, else put a[i] into the map.` `        ``for` `(``int` `i = ``0``; i < arr.length; i++) {`   `            ``if` `(set.get(arr[i] - d) != ``null` `                ``&& set.get(arr[i] - ``2` `* d) != ``null``) {` `                ``count += (set.get(arr[i] - d)` `                          ``* set.get(arr[i] - ``2` `* d));` `            ``}`   `            ``// Update the map.` `            ``if` `(set.get(arr[i]) == ``null``) {` `                ``set.put(arr[i], ``1``);` `            ``}` `            ``else` `{` `                ``set.put(arr[i], set.get(arr[i]) + ``1``);` `            ``}` `        ``}`   `        ``return` `count;` `    ``}`   `    ``// Driver Code` `    ``public` `static` `void` `main(String args[])` `    ``{`   `        ``// Test Case 1:` `        ``int` `a1[] = { ``1``, ``6``, ``7``, ``7``, ``8``, ``10``, ``12``, ``13``, ``14` `};` `        ``int` `d1 = ``3``;` `        ``System.out.println(countTriplets(a1, d1));`   `        ``// Test Case 2:` `        ``int` `a2[] = { ``6``, ``3``, ``4``, ``5` `};` `        ``int` `d2 = ``1``;` `        ``System.out.println(countTriplets(a2, d2));`   `        ``// Test Case 3:` `        ``int` `a3[] = { ``1``, ``2``, ``4``, ``5``, ``7``, ``8``, ``10` `};` `        ``int` `d3 = ``3``;` `        ``System.out.println(countTriplets(a3, d3));` `    ``}` `}`

## Python3

 `# Python3 program to count the number ` `# of triplets from an array.`   `# Function to count the triplets ` `def` `countTriplets (arr, d, n):` `    `  `    ``count ``=` `-``1` `    `  `    ``# Create a map to store (key, values) pair. ` `    ``set` `=` `{}` `    `  `    ``# Traverse the array and check that we ` `    ``# have already put (a[i]-d and a[i]-2*d) ` `    ``# into map or not. If yes we have to get ` `    ``# the values of both the keys and ` `    ``# multiply them, else put a[i] into the map. ` `    ``for` `i ``in` `range` `(``0``, n):` `        ``if` `((arr[i] ``-` `d) ``in` `set``.keys() ``and` `            ``(arr[i] ``-` `2` `*` `d) ``in` `set``.keys()):` `            ``count ``+``=` `(``set``[arr[i] ``-` `d] ``*` `                      ``set``[arr[i] ``-` `2` `*` `d])` `                      `  `        ``# Update the map` `        ``if` `(arr[i]) ``in` `set``.keys():` `            ``set``[arr[i]] ``+``=` `1` `        ``else``:` `            ``set``[arr[i]] ``=` `1` `            `  `    ``print``(count)` `    `  `# Driver Code  `   `# Test Case 1: ` `a1 ``=` `[ ``1``, ``6``, ``7``, ``7``, ``8``, ``10``, ``12``, ``13``, ``14` `]` `d1 ``=` `3` `countTriplets(a1, d1, ``9``)`   `# Test Case 2: ` `a2 ``=` `[ ``6``, ``3``, ``4``, ``5` `]` `d2 ``=` `1` `countTriplets(a2, d2, ``4``)`   `# Test Case 3: ` `a3 ``=` `[ ``1``, ``2``, ``4``, ``5``, ``7``, ``8``, ``10` `]` `d3 ``=` `3` `countTriplets(a3, d3, ``7``)`   `# This code is contributed by Stream_Cipher`

## C#

 `// C# program to count the number` `// of triplets from an array.` `using` `System;` `using` `System.Collections.Generic;`   `class` `GFG {` ` `  `    ``// Function to count the triplets` `    ``static` `int` `countTriplets(``int``[] arr, ``int` `d)` `    ``{` `        ``int` `count = -1;` ` `  `        ``// Create a map to store (key, values) pair.` `        ``Dictionary<``int``, ``int``> ``set` `= ``new` `Dictionary<``int``, ``int``>();` ` `  `        ``// Traverse the array and check that we` `        ``// have already put (a[i]-d and a[i]-2*d)` `        ``// into map or not. If yes we have to get` `        ``// the values of both the keys and` `        ``// multiply them, else put a[i] into the map.` `        ``for` `(``int` `i = 0; i < arr.Length; i++) {` ` `  `            ``if` `(``set``.ContainsKey(arr[i] - d)` `                ``&& ``set``.ContainsKey(arr[i] - 2 * d)) {` `                ``count += (``set``[arr[i] - d]` `                          ``* ``set``[arr[i] - 2 * d]);` `            ``}` ` `  `            ``// Update the map.` `            ``if` `(!``set``.ContainsKey(arr[i])) {` `                ``set``.Add(arr[i], 1);` `            ``}` `            ``else` `{` `                ``set``[arr[i]] = ``set``[arr[i]] + 1;` `            ``}` `        ``}` ` `  `        ``return` `count;` `    ``}` ` `  `    ``// Driver Code` `    ``public` `static` `void` `Main(String []args)` `    ``{` ` `  `        ``// Test Case 1:` `        ``int` `[]a1 = { 1, 6, 7, 7, 8, 10, 12, 13, 14 };` `        ``int` `d1 = 3;` `        ``Console.WriteLine(countTriplets(a1, d1));` ` `  `        ``// Test Case 2:` `        ``int` `[]a2 = { 6, 3, 4, 5 };` `        ``int` `d2 = 1;` `        ``Console.WriteLine(countTriplets(a2, d2));` ` `  `        ``// Test Case 3:` `        ``int` `[]a3 = { 1, 2, 4, 5, 7, 8, 10 };` `        ``int` `d3 = 3;` `        ``Console.WriteLine(countTriplets(a3, d3));` `    ``}` `}`   `// This code is contributed by Princi Singh`

## Javascript

 ``

Output:

```1
0
2```

Time Complexity: O(NlogN), where N is the size of the given array.
Auxiliary Space: O(N),

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