Given an unsorted integer (positive values only) array of size ‘n’, we can form a group of two or three, the group should be such that the sum of all elements in that group is a multiple of 3. Count all possible number of groups that can be generated in this way.

Input: arr[] = {3, 6, 7, 2, 9} Output: 8 // Groups are {3,6}, {3,9}, {9,6}, {7,2}, {3,6,9}, // {3,7,2}, {7,2,6}, {7,2,9} Input: arr[] = {2, 1, 3, 4} Output: 4 // Groups are {2,1}, {2,4}, {2,1,3}, {2,4,3}

The idea is to see remainder of every element when divided by 3. A set of elements can form a group only if sun of their remainders is multiple of 3. Since the task is to enumerate groups, we count all elements with different remainders.

1. Hash all elements in a count array based on remainder, i.e, for all elements a[i], do c[a[i]%3]++; 2. Now c[0] contains the number of elements which when divided by 3 leave remainder 0 and similarly c[1] for remainder 1 and c[2] for 2. 3. Now for group of 2, we have 2 possibilities a. 2 elements of remainder 0 group. Such possibilities are c[0]*(c[0]-1)/2 b. 1 element of remainder 1 and 1 from remainder 2 group Such groups are c[1]*c[2]. 4. Now for group of 3,we have 4 possibilities a. 3 elements from remainder group 0. No. of such groups are c[0]C3 b. 3 elements from remainder group 1. No. of such groups are c[1]C3 c. 3 elements from remainder group 2. No. of such groups are c[2]C3 d. 1 element from each of 3 groups. No. of such groups are c[0]*c[1]*c[2]. 5. Add all the groups in steps 3 and 4 to obtain the result.

## C

#include<stdio.h> // Returns count of all possible groups that can be formed from elements // of a[]. int findgroups(int arr[], int n) { // Create an array C[3] to store counts of elements with remainder // 0, 1 and 2. c[i] would store count of elements with remainder i int c[3] = {0}, i; int res = 0; // To store the result // Count elements with remainder 0, 1 and 2 for (i=0; i<n; i++) c[arr[i]%3]++; // Case 3.a: Count groups of size 2 from 0 remainder elements res += ((c[0]*(c[0]-1))>>1); // Case 3.b: Count groups of size 2 with one element with 1 // remainder and other with 2 remainder res += c[1] * c[2]; // Case 4.a: Count groups of size 3 with all 0 remainder elements res += (c[0] * (c[0]-1) * (c[0]-2))/6; // Case 4.b: Count groups of size 3 with all 1 remainder elements res += (c[1] * (c[1]-1) * (c[1]-2))/6; // Case 4.c: Count groups of size 3 with all 2 remainder elements res += ((c[2]*(c[2]-1)*(c[2]-2))/6); // Case 4.c: Count groups of size 3 with different remainders res += c[0]*c[1]*c[2]; // Return total count stored in res return res; } // Driver program to test above functions int main() { int arr[] = {3, 6, 7, 2, 9}; int n = sizeof(arr)/sizeof(arr[0]); printf("Required number of groups are %d\n", findgroups(arr,n)); return 0; }

## Java

class FindGroups { // Returns count of all possible groups that can be formed from elements // of a[]. int findgroups(int arr[], int n) { // Create an array C[3] to store counts of elements with remainder // 0, 1 and 2. c[i] would store count of elements with remainder i int c[] = new int[]{0, 0, 0}; int i; int res = 0; // To store the result // Count elements with remainder 0, 1 and 2 for (i = 0; i < n; i++) c[arr[i] % 3]++; // Case 3.a: Count groups of size 2 from 0 remainder elements res += ((c[0] * (c[0] - 1)) >> 1); // Case 3.b: Count groups of size 2 with one element with 1 // remainder and other with 2 remainder res += c[1] * c[2]; // Case 4.a: Count groups of size 3 with all 0 remainder elements res += (c[0] * (c[0] - 1) * (c[0] - 2)) / 6; // Case 4.b: Count groups of size 3 with all 1 remainder elements res += (c[1] * (c[1] - 1) * (c[1] - 2)) / 6; // Case 4.c: Count groups of size 3 with all 2 remainder elements res += ((c[2] * (c[2] - 1) * (c[2] - 2)) / 6); // Case 4.c: Count groups of size 3 with different remainders res += c[0] * c[1] * c[2]; // Return total count stored in res return res; } public static void main(String[] args) { FindGroups groups = new FindGroups(); int arr[] = {3, 6, 7, 2, 9}; int n = arr.length; System.out.println("Required number of groups are " + groups.findgroups(arr, n)); } }

Output:

Required number of groups are 8

Time Complexity: O(n)

Auxiliary Space: O(1)

This article is contributed by Amit Jain. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above