# HackerRank ‘Non-Divisible Subset’ Solution

##### Short Problem Definition:

Given a set S of n distinct integers, print the size of a maximal subset S’ of S where the sum of any 2 numbers in S’ are not evenly divisible by k.

Non-Divisible Subset

##### Complexity:

time complexity is O(N)

space complexity is O(N)

##### Execution:

This is by all means not an easy task and is also reflected by the high failure ratio of the participants. For a sum of two numbers to be evenly divisible by k the following condition has to hold. If the remainder of N1%k == r then N2%k = k-r for N1+N2 % k == 0. Let us calculate the set of all numbers with a remainder of r and k-r and pick the larger set. If the remainder is half of k such as 2 % 4 = 2 or exactly k such as 4 % 4 = 0, just one number from each of these sets can be contained in S’.

##### Solution:
```def solveSubset(S, k, n):
r =  * k

for value in S:
r[value%k] += 1

result = 0
for a in xrange(1, (k+1)//2):
result += max(r[a], r[k-a])
if k % 2 == 0 and r[k//2]:
result += 1
if r:
result += 1
return result

n, k = map(int, raw_input().split())
S = map(int, raw_input().split())
print solveSubset(S, k, n)
```

[rust]
use std::io;

fn get_numbers() -> Vec<u32> {
let mut line = String::new();
line.split_whitespace().map(|s| s.parse::<u32>().unwrap()).collect()
}

fn calculate_nondivisible(a: Vec<u32>, n: usize, k: usize) -> u32 {
let mut result = 0;

let mut r = vec![0; k];
for val in a {
r[(val as usize)%k] += 1;
}

for idx in 1..(k+1)/2 {
result += std::cmp::max(r[idx as usize], r[(k-idx) as usize]);
}

if k % 2 == 0 && r[k/2] != 0 {
result += 1;
}
if r != 0 {
result += 1;
}

result
}

fn main() {
let line = get_numbers();
let a = get_numbers();

println!("{}", calculate_nondivisible(a, line as usize, line as usize) );
}
[/rust]

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