# Find the maximum number of elements divisible by 3

Given an array of size **N**. The task to find the maximum possible number of elements divisible by 3 that are in the array after performing the operation an arbitrary (possibly, zero) number of times. In each operation, one can add any two elements of the array. **Examples:**

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

After applying the operation once (on elements 1, 2), the array becomes {3, 3}.

It contains 2 numbers which are divisible by 3 which are maximum possible.Input :a[] = {1, 1, 1, 1, 1, 2, 2}Output :3

**Approach :**

Let cnt_{i} be the number of elements of a with the remainder i modulo 3. Then the initial answer can be represented as cnt_{0} and we have to compose numbers with remainders 1 and 2 somehow optimally. It can be shown that the best way to do it is the following:

- Firstly, while there is at least one remainder of 1 and at least one remainder of 2 then compose them into one 0. After this, at least one of the numbers cnt
_{1}, cnt_{2}will be zero, then we have to compose remaining numbers into numbers divisible by 3. - If cnt
_{1}=0 then the maximum remaining number of elements we can obtain is [cnt_{2}/3] (because 2+2+2=6), and in the other case (cnt_{2}=0) the maximum number of elements is [cnt_{1}/3] (because 1+1+1=3).

Below is the implementation of the above approach :

## C++

`// C++ program to find the maximum` `// number of elements divisible by 3` `#include <bits/stdc++.h>` `using` `namespace` `std;` `// Function to find the maximum` `// number of elements divisible by 3` `int` `MaxNumbers(` `int` `a[], ` `int` `n)` `{` ` ` `// To store frequency of each number` ` ` `int` `fre[3] = { 0 };` ` ` `for` `(` `int` `i = 0; i < n; i++) {` ` ` `// Store modulo value` ` ` `a[i] %= 3;` ` ` `// Store frequency` ` ` `fre[a[i]]++;` ` ` `}` ` ` `// Add numbers with zero modulo to answer` ` ` `int` `ans = fre[0];` ` ` `// Find minimum of elements with modulo` ` ` `// frequency one and zero` ` ` `int` `k = min(fre[1], fre[2]);` ` ` `// Add k to the answer` ` ` `ans += k;` ` ` `// Remove them from frequency` ` ` `fre[1] -= k;` ` ` `fre[2] -= k;` ` ` `// Add numbers possible with` ` ` `// remaining frequency` ` ` `ans += fre[1] / 3 + fre[2] / 3;` ` ` `// Return the required answer` ` ` `return` `ans;` `}` `// Driver code` `int` `main()` `{` ` ` `int` `a[] = { 1, 4, 10, 7, 11, 2, 8, 5, 9 };` ` ` `int` `n = ` `sizeof` `(a) / ` `sizeof` `(a[0]);` ` ` `// Function call` ` ` `cout << MaxNumbers(a, n);` ` ` `return` `0;` `}` |

## Java

`// Java program to find the maximum ` `// number of elements divisible by 3 ` `import` `java.io.*;` `class` `GFG ` `{` ` ` ` ` `// Function to find the maximum ` ` ` `// number of elements divisible by 3 ` ` ` `static` `int` `MaxNumbers(` `int` `a[], ` `int` `n) ` ` ` `{ ` ` ` `// To store frequency of each number ` ` ` `int` `[]fre = { ` `0` `,` `0` `,` `0` `}; ` ` ` ` ` `for` `(` `int` `i = ` `0` `; i < n; i++)` ` ` `{ ` ` ` `// Store modulo value ` ` ` `a[i] %= ` `3` `; ` ` ` ` ` `// Store frequency ` ` ` `fre[a[i]]++; ` ` ` `} ` ` ` ` ` `// Add numbers with zero modulo to answer ` ` ` `int` `ans = fre[` `0` `]; ` ` ` ` ` `// Find minimum of elements with modulo ` ` ` `// frequency one and zero ` ` ` `int` `k = Math.min(fre[` `1` `], fre[` `2` `]); ` ` ` ` ` `// Add k to the answer ` ` ` `ans += k; ` ` ` ` ` `// Remove them from frequency ` ` ` `fre[` `1` `] -= k; ` ` ` `fre[` `2` `] -= k; ` ` ` ` ` `// Add numbers possible with ` ` ` `// remaining frequency ` ` ` `ans += fre[` `1` `] / ` `3` `+ fre[` `2` `] / ` `3` `; ` ` ` ` ` `// Return the required answer ` ` ` `return` `ans; ` ` ` `} ` ` ` ` ` `// Driver code ` ` ` `public` `static` `void` `main (String[] args) ` ` ` `{` ` ` `int` `a[] = { ` `1` `, ` `4` `, ` `10` `, ` `7` `, ` `11` `, ` `2` `, ` `8` `, ` `5` `, ` `9` `}; ` ` ` ` ` `int` `n = a.length; ` ` ` ` ` `// Function call ` ` ` `System.out.println(MaxNumbers(a, n)); ` ` ` `}` `}` `// This code is contributed by @@ajit..` |

## Python3

`# Python3 program to find the maximum` `# number of elements divisible by 3` `# Function to find the maximum` `# number of elements divisible by 3` `def` `MaxNumbers(a, n):` ` ` ` ` `# To store frequency of each number` ` ` `fre ` `=` `[` `0` `for` `i ` `in` `range` `(` `3` `)]` ` ` `for` `i ` `in` `range` `(n):` ` ` ` ` `# Store modulo value` ` ` `a[i] ` `%` `=` `3` ` ` `# Store frequency` ` ` `fre[a[i]] ` `+` `=` `1` ` ` `# Add numbers with zero modulo to answer` ` ` `ans ` `=` `fre[` `0` `]` ` ` `# Find minimum of elements with modulo` ` ` `# frequency one and zero` ` ` `k ` `=` `min` `(fre[` `1` `], fre[` `2` `])` ` ` `# Add k to the answer` ` ` `ans ` `+` `=` `k` ` ` `# Remove them from frequency` ` ` `fre[` `1` `] ` `-` `=` `k` ` ` `fre[` `2` `] ` `-` `=` `k` ` ` `# Add numbers possible with` ` ` `# remaining frequency` ` ` `ans ` `+` `=` `fre[` `1` `] ` `/` `/` `3` `+` `fre[` `2` `] ` `/` `/` `3` ` ` `# Return the required answer` ` ` `return` `ans` `# Driver code` `a ` `=` `[` `1` `, ` `4` `, ` `10` `, ` `7` `, ` `11` `, ` `2` `, ` `8` `, ` `5` `, ` `9` `]` `n ` `=` `len` `(a)` `# Function call` `print` `(MaxNumbers(a, n))` `# This code is contributed by Mohit Kumar` |

## C#

`// C# program to find the maximum ` `// number of elements divisible by 3 ` `using` `System;` `class` `GFG` `{` ` ` ` ` `// Function to find the maximum ` ` ` `// number of elements divisible by 3 ` ` ` `static` `int` `MaxNumbers(` `int` `[]a, ` `int` `n) ` ` ` `{ ` ` ` `// To store frequency of each number ` ` ` `int` `[]fre = { 0,0,0 }; ` ` ` ` ` `for` `(` `int` `i = 0; i < n; i++)` ` ` `{ ` ` ` `// Store modulo value ` ` ` `a[i] %= 3; ` ` ` ` ` `// Store frequency ` ` ` `fre[a[i]]++; ` ` ` `} ` ` ` ` ` `// Add numbers with zero modulo to answer ` ` ` `int` `ans = fre[0]; ` ` ` ` ` `// Find minimum of elements with modulo ` ` ` `// frequency one and zero ` ` ` `int` `k = Math.Min(fre[1], fre[2]); ` ` ` ` ` `// Add k to the answer ` ` ` `ans += k; ` ` ` ` ` `// Remove them from frequency ` ` ` `fre[1] -= k; ` ` ` `fre[2] -= k; ` ` ` ` ` `// Add numbers possible with ` ` ` `// remaining frequency ` ` ` `ans += fre[1] / 3 + fre[2] / 3; ` ` ` ` ` `// Return the required answer ` ` ` `return` `ans; ` ` ` `} ` ` ` ` ` `// Driver code ` ` ` `static` `public` `void` `Main ()` ` ` `{` ` ` ` ` `int` `[]a = { 1, 4, 10, 7, 11, 2, 8, 5, 9 }; ` ` ` ` ` `int` `n = a.Length; ` ` ` ` ` `// Function call ` ` ` `Console.WriteLine(MaxNumbers(a, n)); ` ` ` `}` `}` `// This code is contributed by AnkitRai01` |

## Javascript

`<script>` `// Javascript program to find the maximum` `// number of elements divisible by 3` `// Function to find the maximum` `// number of elements divisible by 3` `function` `MaxNumbers(a, n)` `{` ` ` `// To store frequency of each number` ` ` `let fre = ` `new` `Array(3).fill(0);` ` ` `for` `(let i = 0; i < n; i++) {` ` ` `// Store modulo value` ` ` `a[i] %= 3;` ` ` `// Store frequency` ` ` `fre[a[i]]++;` ` ` `}` ` ` `// Add numbers with zero modulo to answer` ` ` `let ans = fre[0];` ` ` `// Find minimum of elements with modulo` ` ` `// frequency one and zero` ` ` `let k = Math.min(fre[1], fre[2]);` ` ` `// Add k to the answer` ` ` `ans += k;` ` ` `// Remove them from frequency` ` ` `fre[1] -= k;` ` ` `fre[2] -= k;` ` ` `// Add numbers possible with` ` ` `// remaining frequency` ` ` `ans += parseInt(fre[1] / 3) + parseInt(fre[2] / 3);` ` ` `// Return the required answer` ` ` `return` `ans;` `}` `// Driver code` ` ` `let a = [ 1, 4, 10, 7, 11, 2, 8, 5, 9 ];` ` ` `let n = a.length;` ` ` `// Function call` ` ` `document.write(MaxNumbers(a, n));` `</script>` |

**Output:**

5

**Time Complexity : **O(N)

**Auxiliary Space: **O(1)