Function max_sum_subarray_k 

pub fn max_sum_subarray_k(nums: &[i64], k: usize) -> Option<i64>

k uzunlikdagi qism-massivning maksimal yig’indisi.

• Time: O(n), Space: O(1).

Misol

use rust_algorithms::other::sliding_window::max_sum_subarray_k;

assert_eq!(max_sum_subarray_k(&[1, 4, 2, 10, 2, 3, 1, 0, 20], 4), Some(24));
assert_eq!(max_sum_subarray_k(&[1, 2], 5), None); // k massivdan katta