Count of triplets in an array that satisfy the given conditions - GeeksforGeeks

14 May, 2021
Given an array arr[] of N elements, the task is to find the count of triplets (arr[i], arr[j], arr[k]) such that (arr[i] + arr[j] + arr[k] = L) and (L % arr[i] = L % arr[j] = L % arr[k] = 0.Examples:  
Input: arr[] = {2, 4, 5, 6, 7} Output: 1 Only possible triplet is {2, 4, 6}Input: arr[] = {4, 4, 4, 4, 4} Output: 10  
 
Approach:  
Consider the equations below:  
L = a * arr[i], L = b * arr[j] and L = c * arr[k]. Thus, arr[i] = L / a, arr[j] = L / b and arr[k] = L / c. So, using this in arr[i] + arr[j] + arr[k] = L gives L / a + L / b + L / c = L or 1 / a + 1 / b + 1 / c = 1. Now, there can only be 10 possible solutions of the above equation, which are {2, 3, 6} {2, 4, 4} {2, 6, 3} {3, 2, 6} {3, 3, 3} {3, 6, 2} {4, 2, 4} {4, 4, 2} {6, 2, 3} {6, 3, 2}All possible triplets (arr[i], arr[j], arr[k]) will satisfy these solutions. So, all these solutions can be stored in a 2D array say test[][3]. So, that all the triplets which satisfy these solutions can be found.  
 
For all i from 1 to N. Consider arr[i] as the middle element of the triplet. And find corresponding first and third elements of the triplet for all possible solutions of the equation 1 / a + 1 / b + 1 / c = 1. Find the answer for all the cases and add them to the final answer.
Maintain the indices where arr[i] occurs in the array. For a given possible solution of equation X and for every number arr[i] keep the number as middle element of triplet and find the first and the third element. Apply binary search on the available set of indices for the first and the third element to find the number of occurrence of the first element from 1 to i – 1 and the number of occurrences of the third element from i + 1 to N. Multiply both the values and add it to the final answer.
Below is the implementation of the above approach:  
C++
Java
Python3
C#
Javascript
// C++ implementation of the approach#include <bits/stdc++.h>using namespace std;#define ll long long int#define MAX 100001#define ROW 10#define COl 3 vector<int> indices[MAX]; // All possible solutions of the// equation 1/a + 1/b + 1/c = 1int test[ROW][COl] = { { 2, 3, 6 },                       { 2, 4, 4 },                       { 2, 6, 3 },                       { 3, 2, 6 },                       { 3, 3, 3 },                       { 3, 6, 2 },                       { 4, 2, 4 },                       { 4, 4, 2 },                       { 6, 2, 3 },                       { 6, 3, 2 } }; // Function to find the tripletsint find_triplet(int array[], int n){    int answer = 0;     // Storing indices of the elements    for (int i = 0; i < n; i++) {        indices[array[i]].push_back(i);    }     for (int i = 0; i < n; i++) {        int y = array[i];         for (int j = 0; j < ROW; j++) {            int s = test[j][1] * y;             // Check if y can act as the middle            // element of triplet with the given            // solution of 1/a + 1/b + 1/c = 1            if (s % test[j][0] != 0)                continue;            if (s % test[j][2] != 0)                continue;             int x = s / test[j][0];            ll z = s / test[j][2];            if (x > MAX || z > MAX)                continue;             int l = 0;            int r = indices[x].size() - 1;             int first = -1;             // Binary search to find the number of            // possible values of the first element            while (l <= r) {                int m = (l + r) / 2;                 if (indices[x][m] < i) {                    first = m;                    l = m + 1;                }                else {                    r = m - 1;                }            }             l = 0;            r = indices[z].size() - 1;             int third = -1;             // Binary search to find the number of            // possible values of the third element            while (l <= r) {                int m = (l + r) / 2;                 if (indices[z][m] > i) {                    third = m;                    r = m - 1;                }                else {                    l = m + 1;                }            }             if (first != -1 && third != -1) {                 // Contribution to the answer would                // be the multiplication of the possible                // values for the first and the third element                answer += (first + 1) * (indices[z].size() - third);            }        }    }     return answer;} // Driver codeint main(){    int array[] = { 2, 4, 5, 6, 7 };    int n = sizeof(array) / sizeof(array[0]);     cout << find_triplet(array, n);     return 0;}
// Java implementation of the approachimport java.util.*; class GFG{ static int MAX = 100001;static int ROW = 10;static int COl = 3; static Vector<Integer> []indices = new Vector[MAX]; // All possible solutions of the// equation 1/a + 1/b + 1/c = 1static int test[][] = { { 2, 3, 6 }, { 2, 4, 4 },                        { 2, 6, 3 }, { 3, 2, 6 },                        { 3, 3, 3 }, { 3, 6, 2 },                        { 4, 2, 4 }, { 4, 4, 2 },                        { 6, 2, 3 }, { 6, 3, 2 } }; // Function to find the tripletsstatic int find_triplet(int array[], int n){    int answer = 0;    for (int i = 0; i < MAX; i++)    {        indices[i] = new Vector<>();    }         // Storing indices of the elements    for (int i = 0; i < n; i++)    {        indices[array[i]].add(i);    }     for (int i = 0; i < n; i++)    {        int y = array[i];         for (int j = 0; j < ROW; j++)        {            int s = test[j][1] * y;             // Check if y can act as the middle            // element of triplet with the given            // solution of 1/a + 1/b + 1/c = 1            if (s % test[j][0] != 0)                continue;            if (s % test[j][2] != 0)                continue;             int x = s / test[j][0];            int z = s / test[j][2];            if (x > MAX || z > MAX)                continue;             int l = 0;            int r = indices[x].size() - 1;             int first = -1;             // Binary search to find the number of            // possible values of the first element            while (l <= r)            {                int m = (l + r) / 2;                 if (indices[x].get(m) < i)                {                    first = m;                    l = m + 1;                }                else                {                    r = m - 1;                }            }             l = 0;            r = indices[z].size() - 1;             int third = -1;             // Binary search to find the number of            // possible values of the third element            while (l <= r)            {                int m = (l + r) / 2;                 if (indices[z].get(m) > i)                {                    third = m;                    r = m - 1;                }                else                {                    l = m + 1;                }            }             if (first != -1 && third != -1)            {                 // Contribution to the answer would                // be the multiplication of the possible                // values for the first and the third element                answer += (first + 1) * (indices[z].size() - third);            }        }    }    return answer;} // Driver codepublic static void main(String []args){    int array[] = { 2, 4, 5, 6, 7 };    int n = array.length;     System.out.println(find_triplet(array, n));}} // This code is contributed by Rajput-Ji
# Python3 implementation of the approachMAX = 100001ROW = 10COL = 3 indices = [0] * MAX # All possible solutions of the# equation 1/a + 1/b + 1/c = 1test = [[2, 3, 6], [2, 4, 4],        [2, 6, 3], [3, 2, 6],        [3, 3, 3], [3, 6, 2],        [4, 2, 4], [4, 4, 2],        [6, 2, 3], [6, 3, 2]] # Function to find the tripletsdef find_triplet(array, n):    answer = 0     for i in range(MAX):        indices[i] = []     # Storing indices of the elements    for i in range(n):        indices[array[i]].append(i)     for i in range(n):        y = array[i]         for j in range(ROW):            s = test[j][1] * y             # Check if y can act as the middle            # element of triplet with the given            # solution of 1/a + 1/b + 1/c = 1            if s % test[j][0] != 0:                continue            if s % test[j][2] != 0:                continue             x = s // test[j][0]            z = s // test[j][2]            if x > MAX or z > MAX:                continue             l = 0            r = len(indices[x]) - 1            first = -1             # Binary search to find the number of            # possible values of the first element            while l <= r:                m = (l + r) // 2                 if indices[x][m] < i:                    first = m                    l = m + 1                else:                    r = m - 1             l = 0            r = len(indices[z]) - 1            third = -1             # Binary search to find the number of            # possible values of the third element            while l <= r:                m = (l + r) // 2                 if indices[z][m] > i:                    third = m                    r = m - 1                else:                    l = m + 1             if first != -1 and third != -1:                 # Contribution to the answer would                # be the multiplication of the possible                # values for the first and the third element                answer += (first + 1) * (len(indices[z]) - third)    return answer # Driver Codeif __name__ == "__main__":    array = [2, 4, 5, 6, 7]    n = len(array)     print(find_triplet(array, n)) # This code is contributed by# sanjeev2552
// C# implementation of the approachusing System;using System.Collections.Generic; class GFG{ static int MAX = 100001;static int ROW = 10;static int COl = 3; static List<int> []indices = new List<int>[MAX]; // All possible solutions of the// equation 1/a + 1/b + 1/c = 1static int [,]test = { { 2, 3, 6 }, { 2, 4, 4 },                       { 2, 6, 3 }, { 3, 2, 6 },                       { 3, 3, 3 }, { 3, 6, 2 },                       { 4, 2, 4 }, { 4, 4, 2 },                       { 6, 2, 3 }, { 6, 3, 2 } }; // Function to find the tripletsstatic int find_triplet(int []array, int n){    int answer = 0;    for (int i = 0; i < MAX; i++)    {        indices[i] = new List<int>();    }         // Storing indices of the elements    for (int i = 0; i < n; i++)    {        indices[array[i]].Add(i);    }     for (int i = 0; i < n; i++)    {        int y = array[i];         for (int j = 0; j < ROW; j++)        {            int s = test[j, 1] * y;             // Check if y can act as the middle            // element of triplet with the given            // solution of 1/a + 1/b + 1/c = 1            if (s % test[j, 0] != 0)                continue;            if (s % test[j, 2] != 0)                continue;             int x = s / test[j, 0];            int z = s / test[j, 2];            if (x > MAX || z > MAX)                continue;             int l = 0;            int r = indices[x].Count - 1;             int first = -1;             // Binary search to find the number of            // possible values of the first element            while (l <= r)            {                int m = (l + r) / 2;                 if (indices[x][m] < i)                {                    first = m;                    l = m + 1;                }                else                {                    r = m - 1;                }            }             l = 0;            r = indices[z].Count - 1;             int third = -1;             // Binary search to find the number of            // possible values of the third element            while (l <= r)            {                int m = (l + r) / 2;                 if (indices[z][m] > i)                {                    third = m;                    r = m - 1;                }                else                {                    l = m + 1;                }            }             if (first != -1 && third != -1)            {                 // Contribution to the answer would                // be the multiplication of the possible                // values for the first and the third element                answer += (first + 1) * (indices[z].Count - third);            }        }    }    return answer;} // Driver codepublic static void Main(String []args){    int []array = { 2, 4, 5, 6, 7 };    int n = array.Length;     Console.WriteLine(find_triplet(array, n));}} // This code is contributed by Rajput-Ji
<script> // Javascript implementation of the approach var MAX = 100001var ROW = 10var COl = 3 var indices = Array.from(Array(MAX), ()=>new Array()); // All possible solutions of the// equation 1/a + 1/b + 1/c = 1var test = [ [ 2, 3, 6 ],                       [ 2, 4, 4 ],                       [ 2, 6, 3 ],                       [ 3, 2, 6 ],                       [ 3, 3, 3 ],                       [ 3, 6, 2 ],                       [ 4, 2, 4 ],                       [ 4, 4, 2 ],                       [ 6, 2, 3 ],                       [ 6, 3, 2 ] ]; // Function to find the tripletsfunction find_triplet(array, n){    var answer = 0;     // Storing indices of the elements    for (var i = 0; i < n; i++) {        indices[array[i]].push(i);    }     for (var i = 0; i < n; i++) {        var y = array[i];         for (var j = 0; j < ROW; j++) {            var s = test[j][1] * y;             // Check if y can act as the middle            // element of triplet with the given            // solution of 1/a + 1/b + 1/c = 1            if (s % test[j][0] != 0)                continue;            if (s % test[j][2] != 0)                continue;             var x = s / test[j][0];            var z = s / test[j][2];            if (x > MAX || z > MAX)                continue;             var l = 0;            var r = indices[x].length - 1;             var first = -1;             // Binary search to find the number of            // possible values of the first element            while (l <= r) {                var m = (l + r) / 2;                 if (indices[x][m] < i) {                    first = m;                    l = m + 1;                }                else {                    r = m - 1;                }            }             l = 0;            r = indices[z].length - 1;             var third = -1;             // Binary search to find the number of            // possible values of the third element            while (l <= r) {                var m = (l + r) / 2;                 if (indices[z][m] > i) {                    third = m;                    r = m - 1;                }                else {                    l = m + 1;                }            }             if (first != -1 && third != -1) {                 // Contribution to the answer would                // be the multiplication of the possible                // values for the first and the third element                answer += (first + 1) * (indices[z].length - third);            }        }    }     return answer;} // Driver codevar array = [2, 4, 5, 6, 7];var n = array.length;document.write( find_triplet(array, n)); </script>
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Rajput-Ji
sanjeev2552
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