# Count triplets having product 0 from a given array

Given an array arr[] of size N, the task is to count the number of triplets (arr[i], arr[j], arr[k]) such that arr[i] * arr[j] = arr[j] * arr[k] = 0 (i < j < k).
Examples:

Input: arr[] = {0, 8, 12, 0}
Output: 2
Explanation:
Triplets satisfying the given conditions are (0, 8, 0) and (0, 12, 0). Therefore, the required output is 2.

Input: arr[] = {1, 0, 2, 3}
Output: 2

Naive Approach: The simplest approach to solve this problem is to generate all possible triplets from the given array and print the count of triplets that satisfy the given conditions.

Time Complexity: O(N3)
Auxiliary Space: O(1)

Efficient Approach: To optimize the above approach, the idea is to store the count of 0s on the left side and right side of each array element using the Prefix Sum technique. Traverse the array and count the number of triplets that satisfy the given condition by considering the current element of the array as the value of arr[j]. Finally, print the count of triplets that satisfy the given condition. Follow the steps below to solve the problem:

• Initialize an array, say prefixZero[], to store for every index, the count of 0s present in the preceding indices.
• Initialize a variable tripletCnt to store the count of triplets that satisfy the given conditions.
• Traverse the array and check if arr[i] equals to 0 or not. If found to be true, then increment the value of cntTriplet by i * (N – i -1).
• Otherwise, increment the value of tripletCnt by prefixZero[i] * (prefixZero[N – 1] – prefixZero[i]).
• Finally, print the value of tripletCnt.

Below is the implementation of the above approach:

 `// C++ program to implement` `// the above approach`   `#include ` `using` `namespace` `std;`   `// Function to get the count` `// of triples that satisfy` `// the given condition` `int` `cntTriplet(``int` `arr[], ``int` `N)` `{`   `    ``// preZero[i] stores count` `    ``// of 0 up to index i` `    ``int` `preZero[N] = { 0 };`   `    ``// Traverse the array and` `    ``// Count 0s up to index i` `    ``for` `(``int` `i = 0; i < N; i++) {` `        ``if` `(arr[i] == 0) {` `            ``preZero[i]` `                ``= preZero[max(i - 1, 0)] + 1;` `        ``}` `        ``else` `{` `            ``preZero[i]` `                ``= preZero[max(i - 1, 0)];` `        ``}` `    ``}`   `    ``// Stores count of triplet that` `    ``// satisfy the given conditions` `    ``int` `tripletCount = 0;`   `    ``// Traverse the given array` `    ``for` `(``int` `i = 0; i < N; i++) {` `        ``if` `(arr[i] == 0) {`   `            ``// Stores count of elements` `            ``// on  the left side of arr[i]` `            ``int` `X = i;`   `            ``// Stores count of elements` `            ``// on  the right side of arr[i]` `            ``int` `Y = N - i - 1;`   `            ``tripletCount += X * Y;` `        ``}`   `        ``else` `{`   `            ``// Stores count of 0s on` `            ``// the left side of arr[i]` `            ``int` `X = preZero[i];`   `            ``// Stores count of 0s on` `            ``// the right side of arr[i]` `            ``int` `Y = preZero[N - 1]` `                    ``- preZero[i];`   `            ``tripletCount += X * Y;` `        ``}` `    ``}`   `    ``return` `tripletCount;` `}`   `// Driver Code` `int` `main()` `{` `    ``int` `arr[] = { 1, 0, 2, 3 };` `    ``int` `N = ``sizeof``(arr) / ``sizeof``(arr[0]);` `    ``cout << cntTriplet(arr, N);`   `    ``return` `0;` `}`

 `// Java program to implement` `// the above approach` `import` `java.util.*;` `class` `GFG{`   `// Function to get the count` `// of triples that satisfy` `// the given condition` `static` `int` `cntTriplet(``int` `arr[],` `                      ``int` `N)` `{` `  ``// preZero[i] stores count` `  ``// of 0 up to index i` `  ``int` `[]preZero = ``new` `int``[N];`   `  ``// Traverse the array and` `  ``// Count 0s up to index i` `  ``for` `(``int` `i = ``0``; i < N; i++) ` `  ``{` `    ``if` `(arr[i] == ``0``) ` `    ``{` `      ``preZero[i] = preZero[Math.max(i - ``1``, ` `                                    ``0``)] + ``1``;` `    ``}` `    ``else` `    ``{` `      ``preZero[i] = preZero[Math.max(i - ``1``, ``0``)];` `    ``}` `  ``}`   `  ``// Stores count of triplet that` `  ``// satisfy the given conditions` `  ``int` `tripletCount = ``0``;`   `  ``// Traverse the given array` `  ``for` `(``int` `i = ``0``; i < N; i++) ` `  ``{` `    ``if` `(arr[i] == ``0``) ` `    ``{` `      ``// Stores count of elements` `      ``// on  the left side of arr[i]` `      ``int` `X = i;`   `      ``// Stores count of elements` `      ``// on  the right side of arr[i]` `      ``int` `Y = N - i - ``1``;`   `      ``tripletCount += X * Y;` `    ``}` `    ``else` `    ``{` `      ``// Stores count of 0s on` `      ``// the left side of arr[i]` `      ``int` `X = preZero[i];`   `      ``// Stores count of 0s on` `      ``// the right side of arr[i]` `      ``int` `Y = preZero[N - ``1``] - ` `              ``preZero[i];`   `      ``tripletCount += X * Y;` `    ``}` `  ``}`   `  ``return` `tripletCount;` `}`   `// Driver Code` `public` `static` `void` `main(String[] args)` `{` `  ``int` `arr[] = {``1``, ``0``, ``2``, ``3``};` `  ``int` `N = arr.length;` `  ``System.out.print(cntTriplet(arr, N));` `}` `}`   `// This code contributed by gauravrajput1`

 `# Python3 program to implement` `# the above appraoch`   `# Function to get the count ` `# of triples that satisfy` `# the given condition` `def` `cntTriplet(arr, N):`   `    ``# preZero[i] stores count` `    ``# of 0 up to index i` `    ``preZero ``=` `[``0``] ``*` `N`   `    ``# Traverse the array and` `    ``# Count 0s up to index i` `    ``for` `i ``in` `range``(N):` `        ``if` `(arr[i] ``=``=` `0``):` `            ``preZero[i] ``=` `preZero[` `            ``max``(i ``-` `1``, ``0``)] ``+` `1` `        ``else``:` `            ``preZero[i] ``=` `preZero[` `            ``max``(i ``-` `1``, ``0``)]`   `    ``# Stores count of triplet that` `    ``# satisfy the given conditions` `    ``tripletCount ``=` `0`   `    ``# Traverse the given array` `    ``for` `i ``in` `range``(N):` `        ``if` `(arr[i] ``=``=` `0``):`   `            ``# Stores count of elements` `            ``# on  the left side of arr[i]` `            ``X ``=` `i`   `            ``# Stores count of elements` `            ``# on  the right side of arr[i]` `            ``Y ``=` `N ``-` `i ``-` `1`   `            ``tripletCount ``+``=` `X ``*` `Y`   `        ``else``:` `            `  `            ``# Stores count of 0s on` `            ``# the left side of arr[i]` `            ``X ``=` `preZero[i]`   `            ``# Stores count of 0s on` `            ``# the right side of arr[i]` `            ``Y ``=` `preZero[N ``-` `1``] ``-` `preZero[i]`   `            ``tripletCount ``+``=` `X ``*` `Y`   `    ``return` `tripletCount`   `# Driver code` `if` `__name__ ``=``=` `'__main__'``:`   `    ``arr ``=` `[ ``1``, ``0``, ``2``, ``3` `]` `    ``N ``=` `len``(arr)` `    `  `    ``print``(cntTriplet(arr, N))`   `# This code is contributed by Shivam Singh`

 `// C# program to implement` `// the above approach` `using` `System;` `class` `GFG{`   `// Function to get the count` `// of triples that satisfy` `// the given condition` `static` `int` `cntTriplet(``int``[] arr, ` `                      ``int` `N)` `{` `  ``// preZero[i] stores count` `  ``// of 0 up to index i` `  ``int``[] preZero = ``new` `int``[N];`   `  ``// Traverse the array and` `  ``// Count 0s up to index i` `  ``for` `(``int` `i = 0; i < N; i++) ` `  ``{` `    ``if` `(arr[i] == 0) ` `    ``{` `      ``preZero[i] = preZero[Math.Max(i - 1, ` `                                    ``0)] + 1;` `    ``}` `    ``else` `    ``{` `      ``preZero[i] = preZero[Math.Max(i - 1, 0)];` `    ``}` `  ``}`   `  ``// Stores count of triplet that` `  ``// satisfy the given conditions` `  ``int` `tripletCount = 0;`   `  ``// Traverse the given array` `  ``for` `(``int` `i = 0; i < N; i++) ` `  ``{` `    ``if` `(arr[i] == 0) ` `    ``{` `      ``// Stores count of elements` `      ``// on  the left side of arr[i]` `      ``int` `X = i;`   `      ``// Stores count of elements` `      ``// on  the right side of arr[i]` `      ``int` `Y = N - i - 1;`   `      ``tripletCount += X * Y;` `    ``}` `    ``else` `    ``{` `      ``// Stores count of 0s on` `      ``// the left side of arr[i]` `      ``int` `X = preZero[i];`   `      ``// Stores count of 0s on` `      ``// the right side of arr[i]` `      ``int` `Y = preZero[N - 1] - ` `              ``preZero[i];`   `      ``tripletCount += X * Y;` `    ``}` `  ``}`   `  ``return` `tripletCount;` `}`   `// Driver Code` `public` `static` `void` `Main(String[] args)` `{` `  ``int``[] arr = {1, 0, 2, 3};` `  ``int` `N = arr.Length;` `  ``Console.Write(cntTriplet(arr, N));` `}` `}`   `// This code is contributed by Chitranayal`

Output:
```2

```

Time Complexity: O(N)
Auxiliary Space: O(N)

Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the DSA Self Paced Course at a student-friendly price and become industry ready.

Check out this Author's contributed articles.

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.

Article Tags :