Count number of pairs (i, j) such that arr[i] * arr[j] = arr[i] + arr[j] Last Updated : 28 Apr, 2025 Comments Improve Suggest changes Like Article Like Report Given an array arr[] of length N, count the number of pairs (i, j) such that arr[i] * arr[j] = arr[i] + arr[j] and 0 <= i < j <= N. It is also given that elements of the array can be any positive integers including zero. Examples: Input : arr[] = {2, 0, 3, 2, 0} Output : 2 Input : arr[] = {1, 2, 3, 4} Output : 0 Simple solution: We can generate all possible pairs of the array and count those pairs which satisfy the given condition. Below is the implementation of the above approach: CPP // C++ program to count pairs (i, j) // such that arr[i] * arr[j] = arr[i] + arr[j] #include <bits/stdc++.h> using namespace std; // Function to return the count of pairs(i, j) // such that arr[i] * arr[j] = arr[i] + arr[j] long countPairs(int arr[], int n) { long count = 0; for (int i = 0; i < n - 1; i++) { for (int j = i + 1; j < n; j++) { // Increment count if condition satisfy if (arr[i] * arr[j] == arr[i] + arr[j]) count++; } } // Return count of pairs return count; } // Driver code int main() { int arr[] = { 2, 0, 3, 2, 0 }; int n = sizeof(arr) / sizeof(arr[0]); // Get and print count of pairs cout << countPairs(arr, n); return 0; } Java // Java program to count pairs (i, j) // such that arr[i] * arr[j] = arr[i] + arr[j] class GFG { // Function to return the count of pairs(i, j) // such that arr[i] * arr[j] = arr[i] + arr[j] static long countPairs(int arr[], int n) { long count = 0; for (int i = 0; i < n - 1; i++) { for (int j = i + 1; j < n; j++) { // Increment count if condition satisfy if (arr[i] * arr[j] == arr[i] + arr[j]) count++; } } // Return count of pairs return count; } // Driver code public static void main(String[] args) { int arr[] = { 2, 0, 3, 2, 0 }; int n = arr.length; // Get and print count of pairs System.out.println(countPairs(arr, n)); } } Python3 # Python3 program to count pairs (i, j) # such that arr[i] * arr[j] = arr[i] + arr[j] # Function to return the count of pairs(i, j) # such that arr[i] * arr[j] = arr[i] + arr[j] def countPairs(arr, n) : count = 0; for i in range(n - 1) : for j in range(i + 1, n) : # Increment count if condition satisfy if (arr[i] * arr[j] == arr[i] + arr[j]) : count += 1; # Return count of pairs return count; # Driver code if __name__ == "__main__" : arr = [ 2, 0, 3, 2, 0 ]; n = len(arr); # Get and print count of pairs print(countPairs(arr, n)); # This code is contributed by AnkitRai01 C# // C# program to count pairs (i, j) // such that arr[i] * arr[j] = arr[i] + arr[j] using System; class GFG { // Function to return the count of pairs(i, j) // such that arr[i] * arr[j] = arr[i] + arr[j] static long countPairs(int[] arr, int n) { long count = 0; for (int i = 0; i < n - 1; i++) { for (int j = i + 1; j < n; j++) { // Increment count if condition satisfy if (arr[i] * arr[j] == arr[i] + arr[j]) count++; } } // Return count of pairs return count; } // Driver code public static void Main(string[] args) { int[] arr = { 2, 0, 3, 2, 0 }; int n = arr.Length; // Get and print count of pairs Console.WriteLine(countPairs(arr, n)); } } JavaScript <script> // Function to return the count of pairs(i, j) // such that arr[i] * arr[j] = arr[i] + arr[j] function countPairs(arr, n) { let count = 0; for (let i = 0; i < n - 1; i++) { for (let j = i + 1; j < n; j++) { // Increment count if condition satisfy if (arr[i] * arr[j] == arr[i] + arr[j]) count++; } } // Return count of pairs return count; } let arr = [ 2, 0, 3, 2, 0 ]; let n = arr.length; document.write(countPairs(arr, n)); // This code is contributed by khatriharsh281</script> Output: 2 Time Complexity: O(n2) Auxiliary Space: O(12) Efficient Solution: Taking arr[i] as x and arr[j] as y, we can rewrite the given condition as the following equation. xy = x + y xy - x - y = 0 xy - x - y + 1 = 1 x(y - 1) -(y - 1) = 1 (x - 1)(y - 1) = 1 Case 1: x - 1 = 1 i.e x = 2 y - 1 = 1 i.e y = 2 Case 2: x - 1 = -1 i.e x = 0 y - 1 = -1 i.e y = 0 So, now we know that the condition arr[i] * arr[j] = arr[i] + arr[j] will satisfy only if either arr[i] = arr[j] = 0 or arr[i] = arr[j] = 2. All we need to do is to count the occurrence of 2's and 0's. We can then get the number of pairs using formula (count * (count - 1)) / 2 Below is the implementation of the above approach: CPP // C++ program to count pairs (i, j) // such that arr[i] * arr[j] = arr[i] + arr[j] #include <bits/stdc++.h> using namespace std; // Function to return the count of pairs(i, j) // such that arr[i] * arr[j] = arr[i] + arr[j] long countPairs(int arr[], int n) { int countZero = 0; int countTwo = 0; // Count number of 0's and 2's in the array for (int i = 0; i < n; i++) { if (arr[i] == 0) countZero++; else if (arr[i] == 2) countTwo++; } // Total pairs due to occurrence of 0's long pair0 = (countZero * (countZero - 1)) / 2; // Total pairs due to occurrence of 2's long pair2 = (countTwo * (countTwo - 1)) / 2; // Return count of all pairs return pair0 + pair2; } // Driver code int main() { int arr[] = { 2, 0, 3, 2, 0 }; int n = sizeof(arr) / sizeof(arr[0]); // Get and print count of pairs cout << countPairs(arr, n); return 0; } Java // Java program to count pairs (i, j) // such that arr[i] * arr[j] = arr[i] + arr[j] class GFG { // Function to return the count of pairs(i, j) // such that arr[i] * arr[j] = arr[i] + arr[j] static long countPairs(int arr[], int n) { int countZero = 0; int countTwo = 0; // Count number of 0's and 2's in the array for (int i = 0; i < n; i++) { if (arr[i] == 0) countZero++; else if (arr[i] == 2) countTwo++; } // Total pairs due to occurrence of 0's long pair0 = (countZero * (countZero - 1)) / 2; // Total pairs due to occurrence of 2's long pair2 = (countTwo * (countTwo - 1)) / 2; // Return count of all pairs return pair0 + pair2; } // Driver code public static void main(String[] args) { int arr[] = { 2, 0, 3, 2, 0 }; int n = arr.length; // Get and print count of pairs System.out.println(countPairs(arr, n)); } } Python3 # Python3 program to count pairs (i, j) # such that arr[i] * arr[j] = arr[i] + arr[j] # Function to return the count of pairs(i, j) # such that arr[i] * arr[j] = arr[i] + arr[j] def countPairs(arr, n): countZero = 0; countTwo = 0; # Count number of 0's and 2's in the array for i in range(n) : if (arr[i] == 0) : countZero += 1; elif (arr[i] == 2) : countTwo += 1; # Total pairs due to occurrence of 0's pair0 = (countZero * (countZero - 1)) // 2; # Total pairs due to occurrence of 2's pair2 = (countTwo * (countTwo - 1)) // 2; # Return count of all pairs return pair0 + pair2; # Driver code if __name__ == "__main__" : arr = [ 2, 0, 3, 2, 0 ]; n = len(arr); # Get and print count of pairs print(countPairs(arr, n)); # This code is contributed by AnkitRai01 C# // C# program to count pairs (i, j) // such that arr[i] * arr[j] = arr[i] + arr[j] using System; class GFG { // Function to return the count of pairs(i, j) // such that arr[i] * arr[j] = arr[i] + arr[j] static long countPairs(int[] arr, int n) { int countZero = 0; int countTwo = 0; // Count number of 0's and 2's in the array for (int i = 0; i < n; i++) { if (arr[i] == 0) countZero++; else if (arr[i] == 2) countTwo++; } // Total pairs due to occurrence of 0's long pair0 = (countZero * (countZero - 1)) / 2; // Total pairs due to occurrence of 2's long pair2 = (countTwo * (countTwo - 1)) / 2; // Return count of all pairs return pair0 + pair2; } // Driver code public static void Main(string[] args) { int[] arr = { 2, 0, 3, 2, 0 }; int n = arr.Length; // Get and print count of pairs Console.WriteLine(countPairs(arr, n)); } } Output: 2 Time Complexity: O(n) Auxiliary Space: O(1) Comment More infoAdvertise with us I ihritik Follow Improve Article Tags : Mathematical Combinatorial DSA Arrays Practice Tags : ArraysCombinatorialMathematical Similar Reads Basics & PrerequisitesLogic Building ProblemsLogic building is about creating clear, step-by-step methods to solve problems using simple rules and principles. 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