https://www.youtube.com/watch?v=JtMuXmmDl9s. I coded it myself without any hints.

amazon_challenge.rs

use std::cmp;

pub fn find_pairs(numbers: &[i32], k: i32) -> Option<(i32, i32)> {
    let math_mid = numbers.len()/2;

    let mut large_numbers = numbers[math_mid as usize..].iter();
    
    let mut small_numbers = numbers[..math_mid as usize].iter().rev();

    let mut small_numbers_to_larger = large_numbers.clone();

    let mut big_numbers_to_smaller = small_numbers.clone();

    let mut small_number = small_numbers.next()?;
    let mut large_number = large_numbers.next()?;
    
    loop {
        let sum = small_number + large_number;
        
        match sum.cmp(&k) {
            cmp::Ordering::Equal => {
                return Some((large_number.clone(), small_number.clone()));
            },
            cmp::Ordering::Less => {
                match large_numbers.next() {
                    Some(number) => {
                        large_number = number;
                    },
                    None => {
                        small_number = small_numbers_to_larger.next()?;
                    }
                };
                continue;
            }
            cmp::Ordering::Greater => {
                match small_numbers.next() {
                    Some(number) => {
                        small_number = number;
                    },
                    None => {
                        large_number = big_numbers_to_smaller.next()?;
                    }
                };
                continue;
            }
        }
    }
}

mod tests {
    use super::*;

    fn test_pair(array: &[i32], k: i32) {
        let pair = find_pairs(&array, k).unwrap();
        
        assert!(array.contains(&pair.0));
        assert!(array.contains(&pair.1));
        assert_eq!(pair.0+pair.1, k);
    }

    #[test]
    fn first_test() {
        let array = [-1, 2, 5, 8];
        let k = 7;
        
        test_pair(&array, k);
    }

    #[test]
    fn second_test() {
        let array = [-3, -1, 0, 2, 6];
        let k = 6;
        
        test_pair(&array, k);
    }

    #[test]
    fn third_test() {
        let array = [2, 4, 5];
        let k = 8;
        
        let pair = find_pairs(&array, k);
        assert_eq!(pair, None);
    }

    #[test]
    fn fourth_test() {
        let array = [-2, -1, 1, 2];
        let k = 0;
        
        test_pair(&array, k);
    }

    #[test]
    fn fifth_test() {
        let array = [-9, -5, -2, -1, 1, 4, 9, 11];
        let k = 3;

        test_pair(&array, k)
    }
}